Best and easiest way is to buy another tub exactly the same. Fill it up and nute/ph it then just pick the top up off the other tub and place them in the new tub. Then go clean out that tub and do the same for the other.

Other methods include pumps and extra line which probably wouldnt be best for your budget situation. Before I had extra tubs I built a stand out of wood and put a small tarp underneath to catch water... then rested the top on that while I ran and changed the res lol.