Yes and no. They can not take away rights granted to existing licensees. But the copyright holder can reissue the same work under whatever license they wish, without having to modify anything.

What the comment you replied to point out is that if I release something under the GPL and then assign copyright to you, you can legally decide to relicense that code under a proprietary license so that the people buying it from me don't have to abide by the GPL.

If I don't assign copyright, on the other hand, any such third parties would need my permission too.