[synchronise]

Would you have any recourse if he went the other way, and made his code AGPLv3?

You would be unable to use that code unless you moved the entire whole work over to the AGPLv3 license, would you be prepared to do that in order to integrate forked changes?

[__david__]

I'm not sure I understand your question. You can't just take other people's code and change the license willy nilly. Often you can incorporate code with different licenses, but only if the licenses are compatible.

[synchronise]

Well the author could always append AGPLv3 licenses to all of his original code. The parts that he integrated from MicropolisJS would still remain licensed under the GPLv3, but the greater work and resulting binaries would be licensed under the AGPLv3.

[__david__]

True, they could do that. Your original comment said "recourse", as if it would be bad to take GPL code and integrate it into a AGPL project. Which is also part of the reason I don't understand your question—Why would one need any recourse when someone else is abiding by the terms of a license?

[zem]

note that the point of the gpl is not to let the person you forked from reintegrate your forked changes; it's to ensure that no derivative of your work ever locks the end user out. agplv3 is perfectly consistent with that.