If it's not possible to do it with just an upgrade then you'll want to:
1) use the behavior to set the supply of existing units to the new value
2) make your production building train a clone of the original unit type, that has the reduced supply cost simply set in its data, just use requirements to hide the old button and show the new one (a Show requirement)
Cyber Jorts
Trinseno Regular ShmoeIs it possible to change supply cost of a unit with an upgrade?
I have failed so far. I tried using a behavior on the unit with a negative number in Unit: Modification - Supplies. The behavior is on the unit, and the upgrade should alter the supply cost.
A melee mod - The A-move stops here.
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