How to *Lower* an Underground Pop-Up Impact Sprinkler Head
The best way to deal with this does not involve digging up your whole lawn. You just need to dig up the area around the sprinkler head that needs to be lowered.

Once you dig out the area cut off the existing 90 degree elbow that is in there feeding up. Cut on the horizontal pipe about 5 to 6" from the existing elbow.

Next you would install two 45 degree fittings in sequence using a short piece of PVC pipe between them and then another piece to get back to horizontal again. This horizontal run is then terminated with a new 90 degree elbow to face up to accept the sprinkler head. 

Dry fit all the parts before gluing them all together. This is important because you will need to rotate the double 45 degree fittings to get the needed depth. If you need more depth you just need to extend the length of the pipe added between the two 45 fittings. When everything is fit properly make marks with a felt tip marker at each joint so you get them back in the proper position when you take them back apart to glue them together.

Note that if the sprinkler head is fed from an underground TEE instead of just a 90 degree fitting a similar technique can be used to lower the TEE. It just takes two pairs of the 45 degree fittings.