Make something that has instant upgrade time which converts a tank to an airplane model (assuming the engine doesn't crash) that can "upgrade" back to a tank (circular loop).
Requires a bunch of if checkings in the tech though...
Hm...sounds like a nice way to abstract FLOAT systems in Code Geass...too bad the ai won't be able to use the system (since it'll just upgrade forever in a circle). Would require trigger = { ai = no } in the tech.
Basically:
<Land Model>
1 Base Tank 0
2 Base Tank 1
3 Base Tank 2
4 Base Tank 4
....
10 Intermediate Tank (no stats/upgrade time, but prohibitive construction cost, say 999)
<Air Model>
11 Base Flying Tank 0
12 Base Flying Tank 1
....
20 Intermediate Flying (no stats/upgrade time, but prohibitive construction cost, say 999)
1->2->3->4....there is no land tech that unlocks 10 (but 9 can "upgrade" to it).
If tech 2 is unlocked, unlock 2, obsolete 1.
If flying tech 12 is unlocked, obsolete 3,4,5....., and unlock 10 and 12. Obsolete 11. Unit will go from 2->10->12.
Do the same going back.
Going back would be 12->20->2
Once you unlock tech 3, tech 12 would no longer obsolete 3, but only 4, 5, etc.
Requires use of upgrade and noupgrade buttons of course.