Between now and the end of September is good. Rhubarb is a very sturdy plant and can be transplanted pretty much any time without danger of killing it, but doing it in the fall is ideal. It's probably slowing down and approaching dormancy even now.
If it's an old clump be prepared to do a LOT of digging. Rhubard forms a clump of very large roots. I moved a very old, large plant a few years ago and the roots were bigger than my arm. You could have parked a Yugo in the hole I dug!