You have two options. If you are going to replace the rotor with the same type of head, you can just unscrew the top of the head (the "guts") and leave the sprinkler body in the ground. Unscrew the top of the new sprinkler head, and screw it into the old body. That generally only works if you are swapping out the same type of head. If you are going to replace the entire sprinkler, I would suggest digging up the grass around the sprinkler. The area doesn't have to be that big, and it isn't too difficult to do. You will actually have less of a hassle digging up around the sprinkler head and replacing it, than trying to unscrew the sprinkler, and risk having dirt fall into the line or water coming out of the line and backwashing dirt and debris into the line. The easiest option is to buy the same type head, and go with the first option.
Tony Posey
Ridge Run Landscapes