Please do not do that in the future. If you are sure that your imagery is aligned correctly (always remember it could be wrong - compare with GPS traces and adjust if necessary) then it is preferable to modify existing buildings to align with aerial imagery.

If you feel that you absolutely must delete existing buildings and replace them - which is not recommended - then do it in one editing step. Do not delete buildings first and say "I will replace them later". You deleted these buildings 6 weeks ago and you still have not replaced most of them!