How is it legally possible to take a project initially released as open source back to closed source? Especially one licensed with the GPL any version.
There are two things here:
(Mandatory mention: I'm not a lawyer, see yours, and some aspect may be localised and depend on your jurisdiction).
You can't take one user's rights of using given-software v1.5 away once he obtained it trough GPL/OSS licensing.
You can contact the author of given-software v1.5 and
Then you could release further versions (say given-software 2.0) under a commercial license and leave only the previous version free. (as in free speech)
Some OSS projects keep selling new versions, and release the previous one as opensource, at every major version upgrade.
(I'm thinking Ghostscript here, but also Android has been known to do something like that, pre-releasing stuff to interested partners, for hefty prices)
What could go wrong
In those last two situations the only way out of OSS is an hard, huge, gory and sad rewrite of all the contributed code. And even if done right and well, it could still be challengeable, (by that lawyer, yes) so... it really ain't worth it.
I think that if you own all the copyrights to the code base i.e. all contributors have given you (or your company as may be more likely) the copyrights to all of their contributions, then you can re-release that code base under a different licence (which may be a closed source one) if you choose. Some projects (like jQuery) release their code under two different licences simultaneously (one of which is the GPL).
This does not change the licence of any existing versions of the code though and when doing so you may find your contributors feeling quite upset, forking the project and continuing to develop it under a different name. Don't quote me on this but I think that was the kind of thing that has resulted in Libre Office vs. Open Office.
If you are copy right holder of the project, you have right to set (unique) license to each party you distribute your source to.
Now given that you have already given someone a code with GPL, what he/she now possess cannot be revoked unless the code was distributed under some condition.
For example, Open Office was open source (and still is). But since Oracle acquired Sun, people felt that OO might be too tight now so they can begin modifying that code independently under the name of Libre Office and Oracle cannot revoke that right.
However, there are two things you can always do: