Hello everybody!
I had the same problem and it wasn't about read only.
There is another solution to that and you might prefer it to the tagging one. If you simply convert the original files of the disc you've bought / want to change to mp3 files and change the folder where you save it, iTunes won't recognize it anymore as the original album.
Detailed explanation:
1. I copy (better cut, or you'll have it double later) the original m4a files from the bought album into a new folder
2. I convert them into mp3 (free programs, easy to find, like free m4a to mp3 converter), takes about one minute for a whole album
3. I add the converted mp3 files to iTunes (you might want to delete the original files first from iTunes which won't work anymore anyhow, if you did the cut in the beginning)
4. I sync my iPhone. Make sure that, when sync starts and iTunes asks you if you want to transmit new files from your phone (which indeed aren't new, just the ones that are still on your phone from before) to say no, or you'll have everything twice again.
P.s.: if the new folder (where the final mp3 files will be) is organized into artist and album (just create different folders), mine was recognized immediately and sorted correctly. But anyhow, now you can change whatever pleases u in iTunes and it won't change back with synchronization :)