One more thing just occurred to me. If you are re-using your existing heads....while they are removed....this is an excellent time to clean them thoroughly before re-installing.
Included in this is
cleaning out the filter/screen, if it has one. All gear drives have them in the bottom but not sure about your model impact. Here is an schematic that shows one on this model to give you a general idea, but you'll have to figure it out for your particular model:
Don't forget...YT is an excellent site to understand how things work on any particular device, including sprinkler heads.