So, I don't think I am missing something here, but there are units that are not side-grades of other units, but straight-up upgrades of previous ones. I am thinking i.e. about high elves Archers > Archers with armor, or Saurus Warriors > Suarus Warriors with shields, or Tomb Kings Chariots > Chariots with arrows - but there are many more.
So, for this kind of units it would be nice to have a small "Upgrade" button, that lets you upgrade it to its "better" version just paying its differential cost. This way, they would maintain their rank (and name!). I understand that I wouldn't have to recruit them again, so it would be a little bonus in not spending turns to do it, but it would be the same for every faction, so I don't think it would change the gameplay that much. Also, the upgrades are usually small (mostly shielded variants).
So, am I missing something here?
Let me buy shields for my lizard boys!