The problem is that there are now two branches of power plants:
SolarPowerPlant -> FusionPowerPlant -> QuantumPowerPlant,
and the standalone:
and that the AntimatterPowerPlant does not have IsColonyUnique tag.
The SolarPowerPlant is precluded by the AntimatterPowerPlant, but the AntimatterPowerPlant is not precluded by any improvements in the first branch. However if these preclusions would be made then the player would never see why he can not build the AntimatterPowerPlant if one of them is already on the planet. And this is a problem, because then to player must know that to build the AntimatterPowerPlant he would have to tear down those buildings.
The underlying problem is that the xml (data) currently does not support improvement upgrade from multiple buildings. Ideally the AntimatterPowerPlant should be upgrade-able from any power building in the first branch and be buildable when none of these are yet built.
I requested such a change in one of my earlier post (Building uniqueness and limits). By chance i used the powerplant branch as an example (with the fictive NuclearPowerPlant, where the FusionPowerPlant was upgrade-able from either the SolarPowerPlant or the NuclearPowerPlant).
Second (far easier) solution.
Make the AntimatterPowerPlant colony unique and remove him as preclusion from the SolarPowerPlant. However this would allow a regular power plant to be build (and upgraded) on top of a single AntimatterPowerPlant, giving the planet two power plants (three if you also count the SingularityPowerPlant wonder).