Now, when I open my InDesign files, they appear as "filename.indd @ 100% [Converted]" and when I hit Command-S to save, I get sent to the "Save As..." window in a totally different directory from my original file.
If you are using OSX you might checkout the Default Folder utility—it lets you control the Save and Save As dialog directories.
If not, I would give my user an option to overwrite using the older format to avoid any new-version "flakiness."
You've never been able to back save InDesign files you have to first save as IDML to move backwards.
I use this AppleScript to batch convert folders of ID files. The script asks for a folder and converts any ID files to CC2014 and copies the originals into an ~~Archive subfolder: