[ThePythonicCow]

it is impossible to retroactively change a license

Whomever holds the copyright can reissue the code under a different license. Whomever has some rights to the code under an existing license doesn't loose those rights.

So yes, holders of FSF copyright software can continue to use and develop and share that software under the GPLv2 terms.

[kevkrom]

Right. If ALL of the copyright holders of a piece of work agree, the software can be issued under any number of licenses that they choose. However, the fact that multiple licenses exist doesn't invalidate any particular one.

This, by the way, is why one should never assign copyright to the FSF. They claim they want to have the copyrights to make legal enforcement of the license more effective, but it also enables them to do this kind of monkey business as well. Develp and release under whatever license you desire, but always retain your own copyrights so that you have a say in how the software is distributed.