I mean...you don't have to "voluntarily" quit without severance.

You can tell them, in writing, "I am willing and able to continue to perform the tasks I was hired for. If you insist that it be somewhere else, then you can fire me."

You could, and then presumably still not be paid severance despite being fired, and then have to decide whether it's worth trying to fight them legally. I don't pretend to know the right choice for everyone, but when presented with this exact choice, I pretty quickly realized that it wasn't going to be worth the effort. It sucks, but given the option to spend energy on fighting this battle or saving it to put towards finding a new job to support myself and my family (or trying to do both and likely burning myself out from trying to take on more than I could reasonably handle), it wasn't much of a choice. Sometimes being right doesn't mean that you don't lose.