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The problem is, that you have to build right after GAPOWR the Powerup to get the new buildoptions. If you build something between the PowerPlant and its powerup it doesn't work anymore.
This must be why it never seemed to work. I was always under the impression that it was impossible in TS, it was easy to do in RA2 with the PrerequisiteOverride= tag, though. Does this problem also include the construction of units? If you build the upgradable building, a vehicle and then the upgrade does it work?