Do we have a lot of plugins? If it's a change in one or two, it's not a real issue. I don't think I can make this change fully backwards-compatible, and it's worth fixing current Settings api.

I can introduce a temporary (see below) work around this by keeping Setting.set() and replace "update certain fields on create" semantics by simply ignoring updates on create altogether to give plugin authors time to update their code. Ultimately this will have to go though, as the suggested fix introduces a problem with duplicate settings beings silently discarded.