>Instead what people usually do is to say "everything I do from now on is closed source. You can maintain a fork of the old version, but good luck keeping up with my version". Or alternatively "everything I do from now on is under [GPL/AGPL/similar restrictive license], if you want to use it beyond that contact me for a more permissive license deal".

Yes this is what I was describing as reasonable. "Everything after this is governed by X terms" is reasonable. But the whole thing can sound like even if you change terms, previous licenses would still apply, which would be wrong.