When I'm cycling through my units, I'll sometimes notice that a unit is upgradeable to a more advanced unit; often, this happens after I've already moved them away from friendly territory on another task.
Is there a way to list all my upgradeable units, especially after I've finished with the appropriate research?
There is no automated way of doing it, but you do get a large popup whenever a technology is researched, so as soon as you discover something you can go over all the unit types that can now be upgraded. An easy way of doing it is selecting the "unit list" view from the top-left menu (where the current research is usually displayed), then scroll down until you find all the units of the appropriate type - the list is sorted by unit type, so it's easy.
If you don't remember which units upgrade to which, you can take a look at one of these related questions:
Or just be overzealous with checking at least one unit of every reasonable type after each tech is researched :)
