It would be nice not to have to drain the pool, but I bet that is exactly
what your going to do.

There are ways to do these types of things without losing much water.

A couple years ago I had to replace a pressure switch in our spa. It was
located on the main water line right as it passed under the main control
circuit board (a $400 part to replace, if it got wet/ruined). The spa tech
told me it would have to be drained. Instead, we covered the entire circuit
board with plastic wrap, I had LOML standing by as I slowly unscrewed the
old switch. Once the switch was removed, she played "little dutch boy" and
plugged the hole as I prepped the new switch. We only lost a few squirts of
water, and the circuit board stayed high and dry.

Like the other reply mentioned, I'd try to come up with some plastic or
rubber that you can plug the hole from the inside of the pool. Done
properly, the water pressure will hold it on and you won't lose much water.
Worst case, you end up draining the pool, so there's nothing to lose,
really...

-Tim