Can you see the model number and size of the sand filter? Also, do you use a suction cleaner to vacuum the pool? (a cleaner that is attached to the pump suction and roams around the pool when the pump is running). Does the current pump do a reasonably good job of skimming off the surface debris?
Chances are the 1 HP will be fine, but that's just my off-the-cuff opinion and I like to keep the electricity cost down.