Generally speaking, you can't. You can license the whole thing AGPL which is fine; if for some crazy reason you don't want to do that you pretty much have to rewrite anything AGPL in order to license the whole thing GPL3. Why would you do that though?
This is incorrect. You can relicense your code to whatever you want if you and all the license holders agree to do so. You can’t unlicense the old code, though, and if there are more license holders than just you, it’s probably more complicated than it’s worth.
have to rewrite anything AGPL
I’m skeptical this would actually hold up in court because if you wrote the original code and then wrote the new code, it would not be clean room design and would most likely be breaking the terms of the original license.
i guess if you wabt kther gpl licensed projects to uze it? qould make sense if there was an agpl that was compatible with gpl (so all the protections of agpl but the extra ones can be dismissed ONLY for gpl projects)
Doesn't really work that way, I don't think... if you're specifically using agpl, then you don't want people relicensing it as gpl (in particular because that could include Microsoft selling access to a "gpl" version of the project behind an api endpoint or something, and never distributing their source modifications since the gpl doesn't require them to.) I think the gpl-compatible license you're looking for is the gpl.
GPL means you can have the software+source for free and make changes to it. But, if you sell or redistribute it (which you can also do), you have to give people the source code with any improvements you made to it also.
AGPL is the same, but you also have to give people the source if they're accessing it behind a web service or something (i.e. making use of it without technically "receiving" the underlying software).
Yeah, no particular reason. Either it was already licensed GPL3 with multiple authors and not enough reason to "upgrade" to justify the difficulty that entails of consulting with everyone, or it's a type of software where there's basically no difference, or they're just not aware of it.