I don’t believe AGPL can be applied retroactively. What’s there today with MIT license stays and there can be a new version with the AGPL. Unless the author is planning major upgrades, the previous work is open to be forked and used with MIT.
Open source is like free speech. We are never going to control what people can say (as in who uses the sodtware and for what purpose). But we are happy that it exists.
It's not a question of whether it can be applied retroactively, it's whether the existing license is revocable. Open source licenses are not revocable as it would defeat the purpose if copyright holders could simply revoke the original license at will.
Open source is like free speech. We are never going to control what people can say (as in who uses the sodtware and for what purpose). But we are happy that it exists.