Yeah, I hate the non-retroactive-ness of free promotion buildings. The <bapplyfreepromotiononmove>1</bapplyfreepromotiononmove> tag "should" work, and technically does, but seems to cause endless CTD's when I try to edit it in to more buildings. The logs show nothing.
This is probably something that should go into Suggestions, but theoretically every building that improves units should be able to do so retroactively - real military units are constantly training and getting new equipment and reconstituting...and getting new fanatic bloodthirsty recruits fresh from the revivalist megachurch. Not simply with fun promotions like Morale and Combat1 and Ground Weather Reports, but experience. My old 2000 year old arquebusier guy should be able to train to the same combat level as my brand new musketman in the new garrison.
You don't have to say it, I know that putting yet another Fall from Heaven modmod inspired suggestion (passive training) out there would possibly get me being told to start modding properly and put it in myself, but I have no idea how it was done.