Ah. I think longbows should just lose indirect fire on upgrade (actually I don't think they ever need it at all).
There is no way that gatling guns upgraded from Longbows should be getting two free promotions.
My problem with ambush is: doesn't it also affect tanks? MGs should not be good vs tanks, IRL tanks are basically designed as a counter to MGs.
Plus, vanguards are already very weak on attack and MGs are defensive units, so there really isn't a need to use MGs against vanguards.
I'm not sure what the best solution is here. I think CKNs and Longbows should retain some advantage when promoted, but I'm not sure what that should be, and it seems weird to shoe-horn them into a specialization against a particular unit type. Would it be weird if they got morale, as a generic boost, or cover?