To me logical will be cavalry to motorised.
Did to us too.
I would also include Light Armor and Mech. Both seem logical and historical.
Yes.
Hopefully light armor can go to medium armor too-- fingers crossed.
Relax those hands.
This I hope developers considdered in the first place, or did they,since medium armor is standard and by the end of war whenever possible armoured divisions were brought to medium standard.
And some medium armor units were sometimes(Germans in the first place) transformed to heavy armor to exploit expirience and excellence of best crews(and to preserve those skilled crews better in massive tanks).
Medium can go to heavy and heavy to super heavy also.
Unit upgrades are simple to script, so in the unlikely event that not everyone is completely satisfied with the upgrades we have chosen as default, it's just a text edit away.