This is a real question...I fear I may be misunderstanding something. Please correct me if that is the case.
The way it seems to work, is that in the unit designer, if you want to make a unit able to upgrade its equipment at a later time, presumably by buying the upgrade while in owned territory, you have to deliberately select "upgradeable" for that slot. It doesn't appear to increase the cost in any way.
If that is the case, why wouldn't all units be upgradeable by default? Is there a specific reason you wouldn't want to at least have the option for a unit to upgrade?
Thanks!