I think dowgrading units is a good idea. It allows you to adapt to circumstances and I find is realistic. After all, if there is a supply shortage due to few trcuks in the supply chain, why not giving up the trucks in a motorized division? (Just to put an example). Or, if you are running into problems with fuel (so you cant build as many medium or heavy tanks as you would like) why not dowgrading the unit to light tanks? Of course, it should have some drawbacks, like being reduced to zero org and lose of expereince, as the op pointed.