Since the coils are hardwired on the Minelab Excal, your stuck with what ever one came with it. You can send it in to Minelab and have a new coil installed of the size you want.
I prefer the 1000 coil for the hunting I do in fresh and saltwater. It covers a bigger footprint and goes deeper. For diving, I like the 800 loop since it has been lucky for me. Just superstition, but it works for me.