Don't know if this has been noted before -- can't find a thread and/or post about it. When I upgrade from the Fusion Power Plant to the Anti-Matter Power Plant, the Power Plant disappears upon completion, and the tile becomes free to be used for something else. All the same, I still get the bonus to production the power plant is supposed to provide. I have not researched the Quantum Power Plants tech yet, so I don't know if the Anti-Matter plants reappear for the final upgrade.