#226 New tech functions

[Hepster]

Is it possible for tech research to function differently? Instead of immediately changing all the current and all newly bought units to the improved version, can it instead simply unlock the advanced units for purchase?

ie. team roles for long range aircraft and succeeds. All aircraft on the board stay the same however the purchase menu now includes lr fighter & lr bombers at a slightly elevated price. This would slow the effects of more powerful unit improvement techs.