Ending an attunement and attuning to another object in the quickest way.

Let's say a character owns 4 magic items (A, B, C, and D) and is attuned to 3 of them, A, B and C.

They need to end the attunement to object C so to attune to D, use D to cast a spell (let's say Find Familiar) and then re-attune to object C. What is the quickest and most efficient way to do it RAW in terms of short rests and long rests?

Can they do it all during a single long rest?

Does it take 3 short rests? 

I was thinking, during the same short or long rest, another player could attune to object C thus ending the player attunement with it, but can it be done while the player is attuning with object D?

It takes one full Short Rest to attune to an item. It another full Short Rest to unattune to it.

• It would take three Short Rests to switch attunement back and forth.
• I believe a Long Rest can only take the place of one Short Rest.
• I character cannot start attuning to a new item unless they have an attunement slot available.

The first and third bullet points are described in the Attunement section.

• "Attuning to an item requires you to spend a Short Rest focused on only that item while being in physical contact with it (this can’t be the same Short Rest used to learn the item’s properties). This focus can take the form of weapon practice (for a Weapon), meditation (for a Wand), or some other appropriate activity. If the Short Rest is interrupted, the Attunement attempt fails. Otherwise, at the end of the Short Rest, you’re attuned to the magic item and can access its full magical capabilities."
• "You can be attuned to no more than three magic items at a time. Any attempt to attune to a fourth item fails; you must end your Attunement to an item first."

Your could increase the number of attunements.

They must go through the following steps

1. Unattune C
2. Attune D
3. Use D
4. Unattune D
5. Attune C

If they are by themselves, this will take 4 short rests. If they have a buddy available and that buddy has an empty attunement slot, this could possibly be accelerated (rules are not clear if you have have an empty attunement slot when you start attuning, or only when you finish).

An interesting thing I just noticed: a PC who dies and is revivified has all their attunements wiped.

The quickest is 4 Short Rest or 5 if Magic Item D need to be identified first.

Quote from Pantagruel666 >>

They must go through the following steps

1. Unattune C
2. Attune D
3. Use D
4. Unattune D
5. Attune C

If they are by themselves, this will take 4 short rests. If they have a buddy available and that buddy has an empty attunement slot, this could possibly be accelerated (rules are not clear if you have have an empty attunement slot when you start attuning, or only when you finish).

An interesting thing I just noticed: a PC who dies and is revivified has all their attunements wiped.

This is correct. I forgot to count step 4 in my post.

Quote from SmiteMakesRight_3_5 >>

It takes one full Short Rest to attune to an item. It another full Short Rest to unattune to it.

• It would take three Short Rests to switch attunement back and forth.
• I believe a Long Rest can only take the place of one Short Rest.
• I character cannot start attuning to a new item unless they have an attunement slot available.

The first and third bullet points are described in the Attunement section.

• "Attuning to an item requires you to spend a Short Rest focused on only that item while being in physical contact with it (this can’t be the same Short Rest used to learn the item’s properties). This focus can take the form of weapon practice (for a Weapon), meditation (for a Wand), or some other appropriate activity. If the Short Rest is interrupted, the Attunement attempt fails. Otherwise, at the end of the Short Rest, you’re attuned to the magic item and can access its full magical capabilities."
• "You can be attuned to no more than three magic items at a time. Any attempt to attune to a fourth item fails; you must end your Attunement to an item first."

Ending Attunement

Your Attunement to an item ends if you no longer satisfy the prerequisites for Attunement, if the item has been more than 100 feet away for at least 24 hours, if you die, or if another creature attunes to the item. You can also voluntarily end Attunement by spending another Short Rest focused on the item unless the item is cursed.

A Long Rest is a period of extended downtime—at least 8 hours—available to any creature. During a Long Rest, you sleep for at least 6 hours and perform no more than 2 hours of light activity, such as reading, talking, eating, or standing watch.

 You mentioned that a long rest can only take the place of one Short Rest. Can I ask where is this rule from?

This got me thinking, in case you want to keep to long rests only..  if 2 player long rest together, and if player 2 has at least 1 free slot:

• Player 2 could attune to object C in 1 h at the beginning of the long rest and then go to sleep. This would end the attunament of player A with object C.
• Then, toward the end of the long rest, player 1 could wake up and attune to object D.

Then with a second long rest, in a similar but reversed way, player 2 could attune to object D before sleep, then player 1 could attune to object C after sleep.

That would be 2 long rests.

Also, if attunement is considered a light activity, a second player might not be necessary at all, as player 1 could end the attunement from object C and attune to object D during a single long rest. 

Quote from Jayce_Danathar >>

Quote from SmiteMakesRight_3_5 >>

It takes one full Short Rest to attune to an item. It another full Short Rest to unattune to it.

• It would take three Short Rests to switch attunement back and forth.
• I believe a Long Rest can only take the place of one Short Rest.
• I character cannot start attuning to a new item unless they have an attunement slot available.

The first and third bullet points are described in the Attunement section.

• "Attuning to an item requires you to spend a Short Rest focused on only that item while being in physical contact with it (this can’t be the same Short Rest used to learn the item’s properties). This focus can take the form of weapon practice (for a Weapon), meditation (for a Wand), or some other appropriate activity. If the Short Rest is interrupted, the Attunement attempt fails. Otherwise, at the end of the Short Rest, you’re attuned to the magic item and can access its full magical capabilities."
• "You can be attuned to no more than three magic items at a time. Any attempt to attune to a fourth item fails; you must end your Attunement to an item first."

Ending Attunement

Your Attunement to an item ends if you no longer satisfy the prerequisites for Attunement, if the item has been more than 100 feet away for at least 24 hours, if you die, or if another creature attunes to the item. You can also voluntarily end Attunement by spending another Short Rest focused on the item unless the item is cursed.

 

A Long Rest is a period of extended downtime—at least 8 hours—available to any creature. During a Long Rest, you sleep for at least 6 hours and perform no more than 2 hours of light activity, such as reading, talking, eating, or standing watch.

 

This got me thinking:  It is true a long rest can only take the place of one Short Rest.  But if 2 player long rest together:

• Player 2 could attune to object C in 1 h at the beginning of the long rest and then go to sleep. This would end the attunament of player A with object C.
• Then, toward the end of the long rest, player 1 could wake up and attune to object D.

The with a second long rest, in a similar but reversed way, player 2 could attune to object D before sleep, then player 1 could attune to object C after sleep.

Technically, I would say so, but the rules are written with the assumption that attunement completes at the end of the Short Rest so you would need to check that your DM doesn't apply that to Long Rest as well (meaning that Player 2 would complete attunement at the end of the Long Rest regardless of when they started).

Additionally, depending on your DM, you might be able to request completing 2 Short Rests in place of the 2 hours of light activity during the Long Rest if you do not have other requirements on your time.

Also, technically, you didn't mention class requirements and the fastest way to juggle attunements might be to be an Artificer. At level 6, they gain Transmute Magic Item. It is not clear if their attunement carries over to the new replicated magic item, but if that's an option, ask your DM. At the same level, they get Drain Magic Item so if Item D is a replicated magic item, they can drain it instead of spending time unattuning it.

Out of curiosity, do you mind sharing your exact use case? What items are you trying to juggle?

When taking a Long Rest, If you rested at least 1 hour before the interruption, you gain the benefits of a Short Rest which may include Attunement if DM permit. I know i do.

Quote from Plaguescarred >>

When taking a Long Rest, If you rested at least 1 hour before the interruption, you gain the benefits of a Short Rest which may include Attunement if DM permit. I know i do.

That's for "what happens if a long rest is interrupted" -- yes, you get the short rest, but you have to spend extra time to finish the long rest.

Quote from SmiteMakesRight_3_5 >>

Out of curiosity, do you mind sharing your exact use case? What items are you trying to juggle?

Of course. The character is a warlock that has Pact of the Tome. He can not keep it, as they need the evocation for a different thing, but since they have Smith Tools proficiency they want to forge an enspelled dagger of Find Familiar (201 gp)  to keep casting it even after letting go of Pact of the Tome. 

Familiars are not durable, so there might be a situation when the spell must be casted again, but in case all attunement slots are occupied, I was wondering what is the best way to reattune to the dagger, cast find familiar and then go back to the basic 3 items. 

Quote from Plaguescarred >>

When taking a Long Rest, If you rested at least 1 hour before the interruption, you gain the benefits of a Short Rest which may include Attunement if DM permit. I know i do.

 But do you think attuning to an object counts as an interruption for the long rest? I am not sure if it does.

Quote from Jayce_Danathar >>

 But do you think attuning to an object counts as an interruption for the long rest? I am not sure if it does.

 If that text only applies to an interruption, given that every interruption adds 1 hour to the length of time required for a long rest, there's no point, since if you're spending 9 hours anyway you can just take a long rest and a short rest.

Quote from Jayce_Danathar >>

Quote from SmiteMakesRight_3_5 >>

Out of curiosity, do you mind sharing your exact use case? What items are you trying to juggle?

Of course. The character is a warlock that has Pact of the Tome. He can not keep it, as they need the evocation for a different thing, but since they have Smith Tools proficiency they want to forge an enspelled dagger of Find Familiar (201 gp)  to keep casting it even after letting go of Pact of the Tome. 

Familiars are not durable, so there might be a situation when the spell must be casted again, but in case all attunement slots are occupied, I was wondering what is the best way to reattune to the dagger, cast find familiar and then go back to the basic 3 items. 

Ah. Okay, so you have Pact of the Tome for Find Familiar and will be retraining out of the Invocation. I assume that you won't be retraining into Pact of the Chain (superior, more resilient, Find Familiar that can be cast a Magic Action instead of 1 hour). Also, make use of your Familiar Pocket to improve you familiar's survivability.

So, you have an additional problem. You basically also need a Short Rest to cast Find Familiar (60 minutes to 70 minutes to cast). You are out for most of the day.

I assume that the other three magic items are vital to your normal operations? Unfortunately, I don't see a way to cast Find Familiar via magic item other than be a Spell Scroll or an item that requires attunement.