Specifically I'm asking about US usage, as it seems that used Verizon 5c are fetching less in the used market than the pure GSM model. This was discussed a little, back in the 5c/5s rollout thread, ...
Pushing an ota update, even if it is possible on the iPhone, and forcing it upon the owner of a phone is illegal. So if you want to stay unlocked, don't upgrade the phone. It sounds like you're ...