This is NOT solved as it is obvious from the question that the user added building pads first, and then discovered they needed an un-cut existing topo.
If I'm trying to create an 'existing' topo after the fact, how is that done? I've copied the cut topo and pasted in place. I've then changed it to 'existing.' But now I can only select one of the building pads and delete it to return that portion of the topo to its original state. I can't select the others. There seems to be no way to return this copy of the topo to its original state.
I suppose I could find the linked data that was used to make the topo, rename it and linked it back in as existing. But I'm working in a model that was shelved for a couple years, and it is unclear where that data is. This is a completely normal situation to be in in architecture. Unfortunately Autodesk's philosophy seems to be that if a file wasn't managed perfectly from the initial file creation, users are out of luck. Thus the typical forum entry here- where the initial question is ignored (ie how to fix an earlier ovesight) and instead we are told how it should have been done at the outset.
ACTUAL SOLUTION: Eventually I did figure it out through the normal exhausting of every permutation of selection, trial and error. (1) select topo that has been cut, copy and paste in place; (2) select one of the two topos you know have, set to existing; (3) set view to existing in 3d View, delete any sub-regions; (4) select all new phase building pads, copy, paste in place; (5) hide isolate all building pads (remember you'll have dupes, so double the number you started with); (5) select building pad sections one at a time and set to existing; (6) now change iew to existing only, and delete all building pads (obviously they should all be set as existing).
This solution is a little convoluted but it works, and it actually answers the original question. It's the kind of answer you get when the answerer is not incentivized to click 'solved.'
