really dumb question - how to upgrade units?

i'm playing on the xbox360 version and it seems to me the only way i can upgrade a unit (say from a rifleman to a modern infantry) is with leonardo's workshop. i guess elite german units would work too...

but isn't there a way you can pay gold to upgrade outdated units? i always have to end up deleting them instead to maintain a 100% modern army.

You can't unless it's with the German trait or Leonardo's workshop.

This boggled my mind for a long time, too. It's an interesting mechanic, since it means you can't ever really stop building units.