Not obsoleting units?

[Raize]

The nature of the mod I'm working on requires that you be able to upgrade units but also build the older ones (as they don't truly become obsolete).

Does anyone know how to implement this? Right now my highest quality version of the unit replaces all the older options.

 

[Mumin]

Make all units you want to be able to produce but also upgrade upgradeable to a unit which in practice never will be buildable. (E.g. because it requires a tech which is rendered unresearchable.)