The serious thing is prices in this game.
Upgraded M5 costs basicaly as much as upgraded guards and cheaper that trash shocks.
1st unit that needs to cost more is LMG grens. They must cost 280-300 reinforcment wise after they get LMG and vet1 riflenade range.
Right now they can spam it like shit, abusing its 240 MP price. Without any need for expensive infantry.
I don't think changing Reinforcement costs after a unit upgrades/Vets is a good idea or feasible at all. It completely goes against the idea of scaling, unit preservation, etc. Not to mention your MP decreasing as your Pop cap increases so you would soon find yourself with an upgraded army that you can't afford to reinforce.
If anything Grenadiers aren't the problem, its more Commanders like CAS that encourage Gren LMG spam with endless Munis. I still say they should change CAS Muni Swap to be a Supply Drop like Lend Lease or Luf. Supply.