It's certain possible to convert 3D subtitles (.srt) to 3D subtitles (.sup), unfortunately the opposite - sup to srt conversion, is not that simple to reverse the process. Some tools may OCR the 3D subtitles and generate 2D version, but most of the time that creates duplicate entries that you would have to manually delete.
You can always try to download standard subtitles again, instead going with the lengthy process of trying to convert 3D subtitles back to 2D format.Open SUP file Open SRT file
Program for creating and managing movie subtitles
An OCR rippin tool for subtitles