Iirc they are a completely different type of fighter although both are light fighters, so you need to create new air groups and disband the old or relegate to home defense or something. I don´t remember why it was necessary to distinguish them though...

Is there any possibility this might be changed? I hate to lose all the unit experience. The stats for the two German fighter lines are very similar (I don't know if those stats are the same for all countries, or if you customized them). There doesn't seem to be any real purpose to having them, aside from the fact that it is nice to see the different types. The 109 was notoriously short on range, and the later types especially had much lighter armament than the Fw. 190 series, so you could make one line interceptors. I don't think the game engine is really set up to allow fine distinctions like that, though.

Also creating a unit of He 112s, you don't get a historic fighter squadron title, you get 'Messerschmitt Fighters' 1, 2, etc.