Howdy all. First time joining an en world chat. Been on webrpg for many years. Been DMing for over 20 years.
Anyway, I've never had this issue arise. My players never showed the slightest interest in what I did with the PCs after they were done with them. However, this is my take on the issue...
First off, I'm not talking about publishing something with someone else's PC. I'm sure that's wrong.
Secondly, I know it's all a game and we should lighten up.
If your PC's are playing in my world, we share ownership. Once you give up playing in my world, that PC is mine to do with as I please, in my world. You can take him and go play in another world, that's fine. You can request that I kill him off rather than have him continue, and I'll consider that request. But if I want to the PC to be an integral part of my world, that's my right. I'm not rewriting history because you feel I should give up your PC. I'm not stopping you from making a carbon copy of the PC and playing in another game. Go ahead. So don't make me alter my world because you feel an unhealthy attachment to this PC.
One thing that I DO do that I haven't seen mentioned - if a PC becomes an NPC I assume control. BUT, if that NPC is ever encountered by the current group, and that NPC's original creator is in the group, then I let him take control temporarily. In other words, if the current party encounters Tor the barbarian turned innkeeper, and Tor's original creator is now playing a different PC, that player is allowed to take control of Tor and speak and act as he sees fit. I admit, this could be difficult and require extensive briefing in the character has been an NPC for any length of time, but I haven't had that problem yet.