Just screw in the popups, leaving the nozzles off. Then cut off the water supply, and cut the vacuum breaker off, remove the unwanted height, making sure it's 12" higher than the fully popped height of the highest head, and glue it back on with a couple of slip couplins. Wait a few hours for the glue to set up, then turn on the sprinklers and let the line flush, then put the nozzles on, you can do it all in a afternoon. Plant a bush in front of the breaker or buy a lawn jockey.