I think most techs/shops will also change the 1st stage diaphragm when they change the HP seat (it, along with the HP seat, is not part of the "annual" parts kit).
Personally, I just always change the diaphragm if I have one of my MR-22's opened-up for a service.
It is a little expensive to service the MR-22 (compared to some other regs), but it is a great reg (I have two of them).
Best wishes.
Note to the OP: If it is an "older" MR-22, the tech will upgrade the HP seat to the new Tri-Material seat. The "old" seats had problems with longevity, so Mares changed the material used in the seats a few years ago. All the Mares diaphragm regs can be "retro-fitted" with the improved seats.