As noted regularly, upgrading is powerful because it requires no workers.
This diff allows setting modifiers on the upgrading entity while it is being upgraded, which can make the choice to upgrade more tactical.
It introduces upgrade for Cart houses to Appartment, by temporarily lowering population (NB: there's no checking here, so the pop limit can be 'cheated' a bit).
Could/Should also be used for tower upgrades (do we have those?) and so on IMO.
- Would perhaps be nice to set this as a status effect so we can have an icon? 'Tis a bigger diff.