The license is switched from (inherited from original upstream Gitea) MIT to GPL3+. Considering the nature of the software, my first thought was of course "Obviously not for the worse, but why not AGPL instead of GPL?"
Reading through the issue thread on the matter, it seems like the landing of a consensus agreement on GPL3 was after a long and ardous back-and-forth. And even if the member who originally argued for GPL over AGPL have since been enlightened and are now actually prefering AGPL, the existing decision is moving forward to at least lock that part in. There was quite some initial confusion on license "compatibility" (we need better words, this is a major source of confusion in itself) behind prefering GPL, as I read it.
A step forward, but I do hope it will be less than a few years until an AGPL Forgejo.
While new contributions to Forgejo will by under the GPL, old contributions coming from Gitea will still be under MIT. The original Gitea code can be sub licensed to GPL but they must still keep the MIT license around.
Presumably it quite a bit of work to track everything as either MIT + GPL or just GPL. If someone made changes to a file only containing MIT licensed code could they not still claim their contributions as MIT license to the project? Forgejo could of course decline the contribution or sub-license it as GPL but wouldn't it be possible?
AIUI, the answer to your question is "yes" (just as for upstream MIT-licensed Gitea changes being merged in after the fork and license change). The file would be sublicensed and available under GPLv3, with the MIT license maintained. So business as usual?
(Not meaning to imply I have a more complete understanding than you here!)
Reading through the issue thread on the matter, it seems like the landing of a consensus agreement on GPL3 was after a long and ardous back-and-forth. And even if the member who originally argued for GPL over AGPL have since been enlightened and are now actually prefering AGPL, the existing decision is moving forward to at least lock that part in. There was quite some initial confusion on license "compatibility" (we need better words, this is a major source of confusion in itself) behind prefering GPL, as I read it.
A step forward, but I do hope it will be less than a few years until an AGPL Forgejo.