units rebelling in 1.5

When a unit of a certain Doctrine switches sides to another nation that uses another Doctrine, the unit should still have its original Doctrine stats-wise. It only changes its appearance to the new Doctrine due to how the graphics system works (changing it is not as easy). But combat wise it should perform like the old Doctrine. The same happens btw. if you capture a province from another Doctrine that is currently building a unit, it will have the stats of that other Doctrine when it spawns, but the appearance of your own Doctrine.

I did notice though that the unit does switch doctrines when you upgrade it. I upgraded a Pan-Asian level 1 heavy tank into a level 3 soviet tank.

True, once you upgrade it you convert it to your own Doctrine again.