I think InDesign does this to prevent you opening a indd file, perhaps modifying and saving it (as a InDesign CC2018 file), only to find out you cannot open it anymore on a InDesign CC2017 anymore (perhaps on a different computer). It was just the same with previous updates (CS5/CS6/CC2017 etc). I just just Save As... , then point to the original name and remame Untitled X to the original name.