I doubt it will damage anything. So you can sleep better at night, just pull the plug to drain the pan again, and then refill with WS. That will also give you a higher percentage of new fluid. If the fluid has never been changed before, I would probably drive a few days and then drain once more. You might want a new drain plug washer on hand just in case. (NAPA ATM PB2403 fits the U341E trans, but I don't know about other models.)