I'm sure there will be a loose explanation for being allowed to completely change classes and this will be done through another character.
I think this is the sort of thing that's best just left as an unexplained game mechanic as any explanation that tries to make in-world sense of it is likely to be unsatisfying. I think Pathfinder WotR had something like "I and/or my party need training" to a certain NPC, which obviously was a totally insufficient explanation of a feature that let us change not only our class (as per what we hear of the BG3 respec) but change their race and appearance as well. But I guess
something was needed as an option to select, and I don't think trying to add any extra detail would have made it any more plausible.
Some things, I think, just need to be handwaved if you're going to use them in a way that makes no sense, and if it's important to you to have an in-world explanation then either you need to come up with one yourself or just not do it. (Personally, though I kept cocking up my WotR character's build and tweaking it, I never actually changed her race or appearance
)
But yes, to the OP, good point. I wonder how the Oathbreaker will be handled. Like others, I am sure Larian have considered this, but am not sure what they will have done. I'd guess something like not giving you the option to pick any other paladin subclass at any respec once we've committed to the Oathbreaker path, but who knows. Possibly they'll just let us make a nonsense of our roleplay if we like. Personally I wouldn't do that, but I guess it does me no harm if the option is there for others.
