The plastic nipple that the hose fits over is not very strong, especially after 20 years. Therefore, be extra careful when removing the old hose. If it fights you, you may want to consider cutting it away from the nipple.

If you make an error in judgment here, you are looking at a new reservoir.

I once did this with a heater core. The hose decided to be a giant PITA. I, being the stronger of the two of us, won the argument. The copper pipe nipple that extended into the heater core did not.