You cannot relicense code you are not the copyright holder of (ignoring public domain, that's a special case).

Just because it is under a permissive BSD license does not mean that you own the copyright; only that the copyright owner permits you to use the material permissively.

You are still unable to re-license it unless you are the copyright owner (or the copyright owner gives you permission to do so, e.g. by dual-licensing the code).

In practice, there's little reason to license under both BSD and GPL since the BSD is compatible with the GPL on its own.

By the way, that might be what you're thinking of; when you have BSD code, you can integrate it into a GPL codebase, but that's not because you're re-licensing it as GPL, but because the GPL is explicitly meant to be compatible with most other free software licenses. You're still using said code under the terms of the BSD, which allow you to use it alongside GPL code.

Presumably what has happened here is that Facebook's lawyers felt more comfortable being more explicit about the intended licenses, though I can't think of a concrete reason why they'd feel the need to do so.