So, I messed up pretty hard when it came to my 3DS sd card. First off, when switching from a 3DS to a 3DSxl, I didn't realize until too late that I couldn't just pop the card into the new system and have all my stuff there, so when I tried to use it I realized I now had an SD card half full of junk data. Naively, I thought I'd figure out a way to recover it later, so I just left it on there. Now I have a 32 gb sd card that is technically full despite having less than half of it's data usable.
Now, here's the problem. I'm looking to delete all my old data from before I swapped systems without impacting my save files for the games I have now. However, when I view the files on my computer, I don't seem to have any way to discern the old files from the new. If anyone can provide advice for this situation, I would greatly appreciate it. Thank you!