You can always rename a unit using shift-n (PTW).
You can clear the name after you upgrade it, so it will not be cursed to die as a vet.
I give it some clever name like Tojo's Terrors.
Sometimes it's useful to give a short name to units before putting them into an Army (Cav1,2,3) , so you can easily distinguish them on the right-click menu.
The BEST use of the rename is to pick out transports in a stack, so you can pick out the one with move points left when doing ship-hopping.