I’m playing a Monster Slayer in CoS for AL. I have reached 6th level and would like to swap something to pick up Healing Guardian so that my table may conserve short rest hit dice and spell slots between encounters.
I am finding that Zone of Truth is not particularly useful for my character and would like to substitute it for Healing Guardian, but Zone of Truth comes from my Xanathar archetype. Is there an accepted ruling whether archetype spells known may be substituted at level up in AL?
Spells granted by an archetype are additional spells that are always prepared. The only time you can change spells is when a rule says that you can.
Here's two bits of source text for you.
Additionally, when you gain a level in this class, you can choose one of the ranger spells you know and replace it with another spell from the ranger spell list, which also must be of a level for which you have spell slots.
Starting at 3rd level, you learn an additional spell when you reach certain levels in this class, as shown in the Gloom Stalker Spells table. The spell counts as a ranger spell for you, but it doesn’t count against the number of ranger spells you know.
Bolded the important text.
You don't "know" the spell, it's granted to you by a feature. Just like if a feature granted you proficiency in Perception, but you wanted Proficiency in Stealth instead, you couldn't just change it unless the text said (or another skill of your choice). If a feature granted you the ability to cast Speak with Animals at will, you couldn't just choose to change that to another spell. What the feature gives, is what the feature gives. Clerics can't change their domain spells, Land Druids can't change their terrain spells, Warlocks can't change their expanded spell list, and Rangers cannot change their bonus archetype spells.
Edit extra note: If you were in a homebrew campaign, most DMs would allow you to make this switch after discussing your ideas with them, so long as they didn't think you were just trying to power game. AL is not a home game, it's organized play. One organizer cannot allow such a class feature modification. They stick to the rules, and you cannot change a class feature that you'd like better x way by the rules.
Starting at 3rd level, you learn an additional spell when you reach certain levels in this class, as shown in the Gloom Stalker Spells table. The spell counts as a ranger spell for you, but it doesn’t count against the number of ranger spells you know.
They're still spells you know.
The RAI seems to be meant to give you extra spells known while making it clear that they don't impact the originally number. The RAW is that you "learn" these spells but they don't count against "the number" you know.
This implies that you still "know" them, but that they are not to be considered when counting the number you know. Without a higher more specific ruling, this (to me) reads as though I am perfectly within the rules to substitute them (though how it impacts "the number" of spells known becomes nebulous).
Changing a class feature to your own desires is against RAW, and these are class feature spells. I wish you luck bringing this to an AL table, I hope that you prepare otherwise in the case that you are denied, as it is not RAW nor RAI to change class features as they are written.
If you really want to get clarification, tweet at Jeremy Crawford. He often makes clarifications on rulings over Twitter.
It’s not changing a class feature to fit my desires, it would be well within scope of the specific class features as they are written unless “you learn an additional spell” is absolutely not meant to suggest “you *know* an additional spell.”
If it’s specifically intended to suggest you can’t swap these then they are worded poorly. Sticking to the literal RAW the only reason I see that they *shouldn’t* be swapped is that they may count against the number known if they were to be swapped for any other spell (meaning that you’re then effectively only opting out knowing the archetype spells and possibly gaining nothing for it).
The spells you get from your subclass are bonus spells, they don’t count against your number of known spells, at level 6 you should know 4 spells, 1st or second level AND protection from evil and good AND zone of truth. You can also change a spell you know every level, so if you have a spell in your 4 you want to swap for healing spirit you can.
I’m playing a Monster Slayer in CoS for AL. I have reached 6th level and would like to swap something to pick up Healing Guardian so that my table may conserve short rest hit dice and spell slots between encounters.
I am finding that Zone of Truth is not particularly useful for my character and would like to substitute it for Healing Guardian, but Zone of Truth comes from my Xanathar archetype. Is there an accepted ruling whether archetype spells known may be substituted at level up in AL?
No, you cannot swap your archetype spells by the rules, nor in AL.
There’s nothing which suggests this under the ranger entry, the Monster Slayer entry, nor the AL handbook. Is there a source you can cite?
Spells granted by an archetype are additional spells that are always prepared. The only time you can change spells is when a rule says that you can.
Here's two bits of source text for you.
Bolded the important text.
You don't "know" the spell, it's granted to you by a feature. Just like if a feature granted you proficiency in Perception, but you wanted Proficiency in Stealth instead, you couldn't just change it unless the text said (or another skill of your choice). If a feature granted you the ability to cast Speak with Animals at will, you couldn't just choose to change that to another spell. What the feature gives, is what the feature gives. Clerics can't change their domain spells, Land Druids can't change their terrain spells, Warlocks can't change their expanded spell list, and Rangers cannot change their bonus archetype spells.
Edit extra note: If you were in a homebrew campaign, most DMs would allow you to make this switch after discussing your ideas with them, so long as they didn't think you were just trying to power game. AL is not a home game, it's organized play. One organizer cannot allow such a class feature modification. They stick to the rules, and you cannot change a class feature that you'd like better x way by the rules.
They're still spells you know.
The RAI seems to be meant to give you extra spells known while making it clear that they don't impact the originally number. The RAW is that you "learn" these spells but they don't count against "the number" you know.
This implies that you still "know" them, but that they are not to be considered when counting the number you know. Without a higher more specific ruling, this (to me) reads as though I am perfectly within the rules to substitute them (though how it impacts "the number" of spells known becomes nebulous).
Changing a class feature to your own desires is against RAW, and these are class feature spells. I wish you luck bringing this to an AL table, I hope that you prepare otherwise in the case that you are denied, as it is not RAW nor RAI to change class features as they are written.
If you really want to get clarification, tweet at Jeremy Crawford. He often makes clarifications on rulings over Twitter.
It’s not changing a class feature to fit my desires, it would be well within scope of the specific class features as they are written unless “you learn an additional spell” is absolutely not meant to suggest “you *know* an additional spell.”
If it’s specifically intended to suggest you can’t swap these then they are worded poorly. Sticking to the literal RAW the only reason I see that they *shouldn’t* be swapped is that they may count against the number known if they were to be swapped for any other spell (meaning that you’re then effectively only opting out knowing the archetype spells and possibly gaining nothing for it).
The spells you get from your subclass are bonus spells, they don’t count against your number of known spells, at level 6 you should know 4 spells, 1st or second level AND protection from evil and good AND zone of truth. You can also change a spell you know every level, so if you have a spell in your 4 you want to swap for healing spirit you can.