You're allowed to modify it as long as you make it very clear that the new license is distinct from the old one: https://www.gnu.org/licenses/gpl-faq.html#ModifyGPL

That is super interesting, I was actually thinking about it as I was writing that response. Isn't that non-enforceable? As in, if you write a legal document, and then make that document law, and then copyright it, it would mean that you wouldn't be able to modify the law without breaching copyright law. Is this really true and enforceable?

You cannot copyright a license.