Roland Johansen
Deity
Good to know.
So, assuming there is no imminent threat, if I have a few Longbow's garrisoned in cities, its best to wait until they can be upgraded all the way to SAM or Mechanized Infantry rather than bumping them up at each tier (ie Rifleman, then Infantry and so on).
Yes, that's one of the bad side effects of the +20.
However, the power rating of your army is known by the AI civilisations (like you can see theirs in graphs). AI civilisations will only attack you when you are not too much stronger then they are. So a large modern force can stop the AI from declaring war on you even if they don't like you.