So are there any downsides to taking an oath and leaving the Three?

[Argon]

Not totally sure how this oath system works. Is it anything like Covenants?

[Klendt]

Pardon the delayed response.
There are pronounced drawbacks, if you do it too late into the game. There are still drawbacks for doing it earlier, but they're not nearly so bad.

People tend to refer to the creeds offhand as "covenants", and I'd say it's pretty apt, but your Creed is generally a lot more important as it flavors just about everything you do. The goods the merchants sell, the number of renewable items you get on resting at the sanctuary, and the items you can upgrade the creed to provide on resting.

If you were to leave your original creed, all of the NPCs will stop offering you their more advanced services and you'll be unable to freely rejoin until you've obtained a pardon. They won't exactly turn hostile to you for just that, though. That requires a bit more wrongdoing.
Advancing in a creed usually requires a good bit of farming to get the necessary bits back to the Leader NPC, and changing faiths means that all of the sanctuaries you claimed for your original creed won't provide you with any of the good bits. You can convert a sanctuary and all its occupants with a crystal sphere, of course, but those are harshly limited, and of course, you can forcibly expunge a sanctuary, but that'll kill off all the NPCs and waste their idols.

So basically the drawback of creed-hopping gets more steep as the game goes on because you'll have likely put in more work for your creed rank-ups and claimed more sanctuaries in their name, and obtaining a pardon if you want back in gets more expensive the higher your character level is.
Still, creed-hopping is an important part of character progression because some creeds are just objectively better for certain character builds.

[Argon]

Originally posted by Old King Klendt:

Pardon the delayed response.
There are pronounced drawbacks, if you do it too late into the game. There are still drawbacks for doing it earlier, but they're not nearly so bad.

People tend to refer to the creeds offhand as "covenants", and I'd say it's pretty apt, but your Creed is generally a lot more important as it flavors just about everything you do. The goods the merchants sell, the number of renewable items you get on resting at the sanctuary, and the items you can upgrade the creed to provide on resting.

If you were to leave your original creed, all of the NPCs will stop offering you their more advanced services and you'll be unable to freely rejoin until you've obtained a pardon. They won't exactly turn hostile to you for just that, though. That requires a bit more wrongdoing.
Advancing in a creed usually requires a good bit of farming to get the necessary bits back to the Leader NPC, and changing faiths means that all of the sanctuaries you claimed for your original creed won't provide you with any of the good bits. You can convert a sanctuary and all its occupants with a crystal sphere, of course, but those are harshly limited, and of course, you can forcibly expunge a sanctuary, but that'll kill off all the NPCs and waste their idols.

So basically the drawback of creed-hopping gets more steep as the game goes on because you'll have likely put in more work for your creed rank-ups and claimed more sanctuaries in their name, and obtaining a pardon if you want back in gets more expensive the higher your character level is.
Still, creed-hopping is an important part of character progression because some creeds are just objectively better for certain character builds.

Thanks! Another question, is there a way to replace an NPC with another NPC, like switching out a blacksmith with a sellsword or something along those lines?

[Klendt]

'Fraid not. It's best to plan ahead for your one-stop-shopping needs. At least Calling Horns are so cheap and Guide idols are so common.