Just thinking here, but maybe this would be the prime time to find yourself a valve of sorts so you can negotiate a change more easily the next time.
You'd just have to match the thread size, pitch, and depth with the appropriate seal. Then you could install a hose to drain and then refill.
I'm not sure if that's feasible for this, but if I were doing it that would be something I would research to determine if it was A. Feasible, and/or B. worthwhile.
Maybe it is, or maybe it isn't..... Just thinking...