Installing a lower HP motor would simply cause a motor burn-out; motor load on pool pumps is determined mostly by the impeller . . . and not at all by the motor.

If you have a 1 HP impeller, you can use a 1 HP motor, OR a 2 HP motor. The 1 HP motor might be slightly more efficient, but the 2 HP motor would work just fine. Do it the other way, matching a 2 HP impeller with a 1 HP motor would simply result in burning the motor up, as the 'small' motor tried to turn the 'big' impeller.

You *may* be able to downsize your pump, by installing a downsized impeller, plus a new seal. It depends on the exact model of your pump.