The M3 Grant/Lee's were turned into M7 Priests.....lots of them.
Actually I think most or all of the M7s were new production, not conversions of extant platforms. The old Grants & Lees, Brit and U.S., went to training units (and maybe some to the Yugoslavs via the Brits?)
You forget to mention the M4. The M3 HT's were all used/upgraded to the M5. The M3 Stuart was simply upgraded to the M5. The 37mm ATG was thrown away to the Marines. The French/British/Soviets all used front line M4's not hand me down stuff. The American Allies all got front line equipment at the time it was given. When the Soviets/British got M3's the US was using them too. When the US upgraded to the M4 the Soviets/ British and French all got them too.
The TD HT's were simply phased out. There weren't that many of them to begin with.
I never mentioned the M4 because it was never replaced. The SPA and TD HTs were indeed expedients and were phased out, but they did exist and were used. The M5 version of the HT was just an International Harvester version produced to supplement normal production. The upgrade kits were the A1 kits (M3A1, M5A1) - the pulpit mounts for the M2 HMG. The M3 in its APC role was also never replaced. The M3 Stuarts and the M5 Stuarts were very separate, very distinct vehicles. As you say, the 37mm stayed in service with the Marines but was phased out of the Army.
A good example are the US TD"s. The M-10, M-18 and M-36 all served together for a good part of the war.
Good Hunting.
MR
A good example of...? I think the fact that there's a list of U.S. kit that was used, then discarded or given away, at least partially invalidates your point that the U.S. hadn;t fought long enough to collect junk.
Another facet of that question is U.S. production methods. We didn't produce 4 brands of medium tanks, a gajillion light tanks, several heavies, and a bewildering array of SPGs - we stuck with one or two models of each and ran up the production lines. As has been pointed out by a million other people, this was occasionally to our detriment because the need to keep production at full steam meant we were less willing to consider or adopt needed imrovements in tank/TD design (for instance). We had a crappy jungle boot for a while too, if I remember my reading correctly.
My favorite never-produced conversion is the M36 turret on a M18 chassis. War ended before it was final-proofed, but that would have been a real corker.
-dale