The entire concept of Oath Breaker seems a bit of a stretch to be honest. So lets work with the current premise. A Paladin is someone with such a firm conviction about whatever they have given their oath to, that powers have resulted from this conviction. (The removal of the Holy Warrior makes no sense but this is what we have)

Now during the course of adventuring, your commitment to your Oath has waned and eventually you fall from that Oath. That means you lose those special abilities, right? Evidently not because somewhere the same level of crazy commitment has developed over night for now an evil commitment. This insane commitment has come on so hard and fast that you instantly transform into a new set of powers. But hey for a few thousand gold pieces the powers can be switched back.

I am sorry but for a game that has good, solid lore and story lines does this not seem insane? I am not opposed to the concept of an anti-Paladin, there should be such a class and I am okay with it being a sub class. However to force the change on my character with no real way to know the change is coming until it happens seems insane. For someone to make a Paladin just to purposefully work to become Fallen seems insane, why not start that way?