This is a brutalist example (as internationalist buildings like Seagram and Lever House are already seen as classics) that was once thought as ugly but is now adored by many: Bank of Georgia Headquarters in Tbilisi
Before "ugly modern building":
After: (An architectural classic)
If a building looks old and run-down, most everybody will dislike it. If a building shoulld be renovated, it should be done with higher quality materials that do not appear different to the architect's original plan for the building (unless the solution is completely avant-garde, like the transformation of Tour AXA to Tour First)