You may need a trash pump to get the last bit of water out of the pond. And it might be very valuable to get that last bit!

If you are re-starting your pond with new fish populations, it will usually be much cheaper and easier to manage it with ONLY the fish you have introduced.

Draining it completely dry for several months would work, or you may have to exterminate with a rotenone treatment.

Of course, all of that may be pointless if the water source for your pond can easily bring in undesirable fish species.

Good luck on your pond renovation project!