Nope, didn't know that. Interesting. I don't upgrade units though. I just make sure that each province that has been designed to produce a given unit type produces that unit type each and every turn. After a battle, I merge units that are not up to maxiumum strength, and then pull out the partial units into a reserve group, which I garrison one province behind the front lines. I then us them to top off partial units after subsequent battles. This group of partial units comes in handy sometimes to temporarily garrison a newly taken and turbulent province when my main army moves on to further my attack.

I've always thought it disruptive to my attack flow to transport units back and forth for upgrades. I just us them as is, and upgrade by merging in new, better units as I produce them.