No, they can't do this unless they have copyright assignment from all the authors. If I contributed code to Wordpress under the GPL, I would expect that that code be GPL'd forever. Wordpress can only change the license if they remove my code. (I haven't contributed to Wordpress, but other people theoretically have.)
Hence why I said a new version of wordpress where previous versions of modifications and themes are non functional. Hence if you wish for your product to be ported to a new version you would need to agree to the new terms.
If they haven't rewritten Wordpress to deal with the plethora of obvious technical problems, it seems unlikely that they would rewrite it for mildly-obscure legal reasons.
