Air Upgrades Kaiserreich for Hearts of Iron IV.
Convert old obsolete interwar planes into less useless planes. No longer will you be forced to destroy crappy old planes, instead you have learned that they hold valuable resources you can use to make newer planes with the magic of the convert button. Convert interwar fighters to fighter 1’s, fighter 1’s to fighter 2’s, and all that. No fighter 3’s to Jetfighters though, that would just be silly. Upgrade your Transport Planes as well. And if you have La Resistance convert Scout Planes as well.