Just a thought I had.

An “upgradedby” tag. Essentially, have upgrading be triggered by another unit rather than the unit being upgraded itself; construction units “build” onto the upgraded unit.

For instance, you’d have an Engineer “upgrade” a Vehicle Yard into… a better Vehicle Yard.

There is no inherent benefit to doing this that I can think of other than to be “different” than the run-of-the-mill upgrade lua code. It would be interesting to add more capability to engineer units (obviously this doesn’t apply to certain upgradable units like fixed MGs and towed gun emplacements) so that they have more to do besides building things.