if you own the copyright to a peice of code, you are free to relicense it however you want, at any time.
Existing code in the wild will keep the license it was distributed with, but nothing obliges you to keep updating it.
Existing code in the wild will keep the license it was distributed with, but nothing obliges you to keep updating it.
2) The old code was GPL, so the new one (unless they rewrote it) is a derived work.
I'm not sure the second one applies if it's the original author who relicenses it, but I'm reasonably sure it's at least a legal grey area, and the first one would still be a thing.