How do replace the drain cover without draining the pool?

Get the old one out and bring it to a pool dealer that can get you the right covers. They just screw in and out with a couple of screws for the most part, so you'll have to either Scuba dive or be pretty good at holding your breath to get them out. You'll need weight as well to be able to apply enough weight to get the screws out.

Most pool service companies can do this as well for a fee.

David