This would be a small buff, because players skip half the build power required to first tear down the structure, and then rebuild it, as well as needing some storage and being able to continuously produce energy while the building is being replaced instead of losing out on that income while the first structure is taken down. But I think this would be a nice quality of life change despite the fact that it would be a small buff.
The biggest reason why I'd like this is on Supreme Strait. I like going for the advanced geo first, and adding AA. But if my sea player loses, it would be nice to convert that Advanced Geo into a prude so the longbows don't destroy half my base when they shoot it. Right now you have to take the plant down, and rebuild it from scratch, which I do. But it would be nice to be able to convert geo structures into other geo structures in place.
While I'm suggesting in place building for Prudes >>> Geo and vice versa, the Cortex geo cannon would also be nice to be able to convert back and forth into geo power. This suggestion doesn't have to be limited to just Prudes.
I don't know if this makes them too powerful, being able to convert between defense, eco, safe eco on a whim since players will likely start putting construction turrets nearby to facilitate quick changes as part of their strategy. I can see this being a bit annoying to deal with on maps like Forge, where Cortex players will convert between cannons and Geos to push/defend and eco when they are safe. But I'd like to hear peoples thoughts.