If it was already published as GPL, then they cannot change that. (Otherwise they would have
done so already). That's the nice thing about free software.
BTW: look at the Tentacles of Evil test in
. We try to assume the worst. We try to assume that the copyright owners
will suddenly become a second Caldera/SCO. The software must remain usage,
distributable, modifiable etc. even in that case.
If a software fails that test it won't make it into Debian.
In fact, there are those who claim that the CDDL does not qualify because it fails exactly
that test (due to the choice of venue clause).