I'm pretty lenient when it comes to the mechanical implementation of a PC. All I ask is that you talk to me and we'll work something out.
That doesn't mean there won't be repercussions. Let's say you're a warlock of the great old one. As the campaign goes on, you realize that you really don't want to support their goals at all. So I would prefer that you give me a heads up, but if you decide to completely reject your patron then you're going to be cut off from the source of your power.
So the question becomes, what to do about it. If the GOO is aware of the PC, they get cut off which is what would happen in my game because of the nature of the patrons I allow. If the GOO is unaware, then there's a risk of being noticed. You really don't want Cthulhu noticing you. I don't allow that kind of patron because it has potential repercussions outside of the player. I mean, I may get annoyed and swat a mosquito. I may also decide that I've had enough of these mosquitoes and destroy their habitat. In terms of a GOO that could mean anything from driving he warlock batshit insane to a far spread catastrophe that I just don't want to deal with.
So in my games that would mean either you come in with a new PC, we work out some kind of rebuild, get back in good graces somehow or you find a new patron. Whatever makes sense to the player. You aren't going to be permanently nerfed no matter what. But until it's resolved you don't have a source of power because a warlock's powers are not their own.