Solid plan, except maybe one thing. You have to match the in and the out perfectly, and gently. The pump can't remove water faster than you can add it, or the exchange won't work. If there is no way to dial down the pump, or add more fill hoses, then you may have to just siphon the water out and replenish with one hose. Or siphon with more hoses or a larger hose, as long as you can keep up with the filling.