Accidental compression of (some of?) MP3 files

Really hoping someone can help with this.

Recently, while supposedly doing a disk clean up / defrag, it seems that I accidentally compressed some - but not all! - of my MP3 files.

I have 7,500 songs - all saved in their different album folders. As I only keep the three or four truly decent songs from any one album, that's a lot of different album folders.

As time has gone on and as the capacity of computers has increased, I moved away from 160kbit files to the maximum of 320kbit - so the files in my iTunes folder (and library) should be different sizes, reflecting the different quality that I converted them to, from the original WAV files.

I'm really worried about these slightly separate issues:

i) Have I accidentally compressed all my MP3s to some terrible quality file, like 64kbit / 128kbit / 160kbit? In the actual iTunes application, in the 'Bit Rate' column, the files (according to the iTunes software) are still showing as different quality file sizes, with a great many still at 360kbit (i hope!!). I know that a small bit rate - once converted - cannot be re-converted as the data is irrecoverably lost, so this is a worry.

ii) How can I 'de-compress' the affected files, which are showing in blue font in Windows Explorer? I understand that if I click in each folder, select the files, right click to 'Properties' and click 'Advanced', then (and only then) can I uncheck the 'Compress to save disk space' box. The problem with this is that if I do it manually it will take weeks to go into every folder and repeat the process. I can't face doing it this way. Also, not all the files were compressed in the first place (although many seem to have been), so I'd probably be opening up and going through lots of folders unnecessarily. There are more than 2,000 folders. Surely there must be an easier way? If I "select all" folders and try to uncheck 'compress to save disk space', the option isn't available - presumably because many files were NOT compressed.

Please let me know how to restore the files to their healthy black font and uncompressed state.

Thank you, Kathryn