Possible upgrade path:
Lend-lease tech tree:
Tier III: Stuart I/II (were they fitted with UK radios?)
Tier IV: Stuart V/VI AND Lee/Grant
Tier V: Sherman V (VC Firefly top config). Starter turret with narrow 3" gun shield, top turret with wide 3.5" and 2" gun shield
Tier VI: Sherman I Hybrid/Composite
While for all intents and purposes I and V were same except for the engine, but then Tier VI Sherman I could atleast have later transmission cover (single piece cast) - from pics I saw, all Sherman V's had the older 2" 3-piece lower front whereas Shermans I of later version (esp. with Composite hull - front cast) had the single-piece tranny cover as well.
The light branch may be completely dropped out, with the LL branch starting with M3 Medium/Lee/Grant at Tier IV.
...just some idle thoughts.
EDIT: A problem comes to my mind now: Firefly had just 4 crewmen, right? Dunno if mounting a new weapon can reduce number of crew, otherwise the upgradable fireflies would have simply to live with "ahistorical" co-driver/radio operator (for game mechanics) in the hull