Tasha's Cauldron of Everything (p8) did introduce optional rules for character's changing their subclass - but whether a DM allows it is their decision.
If you don't like the patron you have chosen then have a chat with the DM and describe why you don't like playing it. Since the point of playing D&D is to have fun, most DMs will work in some sort of change in the story to accommodate the change you want to make in the character (as long as you don't do it too often - if you want one patron this week, a different one next week and so on then most DMs get less sympathetic the more often you want to make changes). However, DMs want players to have fun since a big part of the joy of DMing is the fun that everyone derives from the game.
So, if you want to make a change like this or if something in the story makes you think it would fit the character better and you'd enjoy playing more as a result - then talk to the DM and see what you can work out.
It's really more of a story issue than a mechanical issue. As was said above, if you leave a warlock pact, you would obviously give up the features associated with that pact and potentially any eldritch invocations that had a prerequisite of that pact. Beyond that, it's between you, your patron, and your DM.
We had a Warlock that started off as Old One... and we destroyed a town and did some other things... until he wound up going through a portal straight to Hell... where a very helpful litigation devil helped him sign off on a new Patron with some new cool toys. Of course, he didn't get the fine print until he started using the new toys and found out there were some costs associated with some of it. A few more mistakes later and he is now on a Quick Flight Path to his Ever-After... no longer does he get Death Saves... he just punches his ticket. It's odd the conundrum this has provided the party... and who is willing to help him send people through his little wand of portal magic in the hopes of making his pact more favorable. He's gained three levels since then... so, we're still seeing how it's going. Ironically, the Healer of the party was the one that cashed in the chips... so his life is about to get a LOT more stressful...
If your DM says OK, and is willing to work with the changes. But you better have a REALLY good reason, and I would expect repercussions from the spurned Patron.
I believe you may be conflating changing your pact with changing your patron (subclass).
You make a pact with the patron. If you break the pact, you no longer have a pact with the patron. The Pact Boon, you receive at third level is a bonus to your pact, not the pact itself.
The Tasha's rules for changing your subclass deal with changing your patron, not your pact boon. OP asked about spells changing, so it's reasonable to assume they are asking about changing patron, as the patron is what grants your spells. As for how long it takes, it depends:
Either, you follow the "Training rules", and it takes 2*(new level) days, so either 12, 20, or 28 days, and either 600, 1,000, or 1,400 GP.
Or you use the "Sudden change" version, which for warlocks may be a more appropriate choice.
That's my point. I don't believe there is a framework for changing from, say, Pact of the Tome to Pact of the Blade. Although if you did this, surely you would lose the additional cantrips granted by the Tome. You would also lose the ritual casting functionality from the Book of Ancient Secrets eldritch invocation, which has a prerequisite of pact of the tome, so your spells would change there as well.
Oh! I totally read that differently and never considered it in our context. I read it as letting you swap out features within your pact boom rather than allowing a change of pact boom entirely.
What happens if you do, do spells changes, how long does it take?
As Lyxen said it is entirely up to the DM.
Tasha's Cauldron of Everything (p8) did introduce optional rules for character's changing their subclass - but whether a DM allows it is their decision.
If you don't like the patron you have chosen then have a chat with the DM and describe why you don't like playing it. Since the point of playing D&D is to have fun, most DMs will work in some sort of change in the story to accommodate the change you want to make in the character (as long as you don't do it too often - if you want one patron this week, a different one next week and so on then most DMs get less sympathetic the more often you want to make changes). However, DMs want players to have fun since a big part of the joy of DMing is the fun that everyone derives from the game.
So, if you want to make a change like this or if something in the story makes you think it would fit the character better and you'd enjoy playing more as a result - then talk to the DM and see what you can work out.
It's really more of a story issue than a mechanical issue. As was said above, if you leave a warlock pact, you would obviously give up the features associated with that pact and potentially any eldritch invocations that had a prerequisite of that pact. Beyond that, it's between you, your patron, and your DM.
"Not all those who wander are lost"
I believe you may be conflating changing your pact with changing your patron (subclass).
"Not all those who wander are lost"
We had a Warlock that started off as Old One... and we destroyed a town and did some other things... until he wound up going through a portal straight to Hell... where a very helpful litigation devil helped him sign off on a new Patron with some new cool toys. Of course, he didn't get the fine print until he started using the new toys and found out there were some costs associated with some of it. A few more mistakes later and he is now on a Quick Flight Path to his Ever-After... no longer does he get Death Saves... he just punches his ticket. It's odd the conundrum this has provided the party... and who is willing to help him send people through his little wand of portal magic in the hopes of making his pact more favorable. He's gained three levels since then... so, we're still seeing how it's going. Ironically, the Healer of the party was the one that cashed in the chips... so his life is about to get a LOT more stressful...
If your DM says OK, and is willing to work with the changes. But you better have a REALLY good reason, and I would expect repercussions from the spurned Patron.
That's my point. I don't believe there is a framework for changing from, say, Pact of the Tome to Pact of the Blade. Although if you did this, surely you would lose the additional cantrips granted by the Tome. You would also lose the ritual casting functionality from the Book of Ancient Secrets eldritch invocation, which has a prerequisite of pact of the tome, so your spells would change there as well.
"Not all those who wander are lost"
Oh! I totally read that differently and never considered it in our context. I read it as letting you swap out features within your pact boom rather than allowing a change of pact boom entirely.
"Not all those who wander are lost"
They are the same. Yes there are subclasses called pact but it is part of the same thing.
In fairness, I wrote that post almost four years ago. But rereading it, I stand by what I said, at least in the context of when it was written.
"Not all those who wander are lost"