There are no I-20's with 1" inlets.. they are all 3/4". Are you sure it is not an I-25, I-40, or I-60?? They all have the 1" inlet.
Deciding what sprinkler to change to is more about your spacing and GPM requirements than your inlet size. The inlet size can always be changed... The spacing is hard to change without digging up your yard. Do you know what is your max GPM per zone you can use?? How many sprinklers are you wanting to change??