What would people think if units carrying firearms were changed from melee to ranged units with the range starting out as 1 hex, and ranged strength = melee strength? Then your promotions wouldn't be wasted regardless of whatever early-era unit it might have been upgraded from, and help deal with the anomaly that a bowman can kill an enemies that a man with a gun can't (and yes, still true with bow range = 2 and gun range = 1, but still...).