Zune steals our money

I paid for large amount of music, buying credits to purchase singles. The format of the music was changed from wma files to mp3 files, and they removed my usage rights for music I had paid for. When I called them, they had me list every song that was missing from my collection, which was alot, they informed me that there was nothing they could or would do to help me, and they were not required to give me a refund or any credit or compensation of any kind. In my book, this is stealing. They take my money, and then remove my property, which I paid for. They then tell me they don't have to do anything about it. This is not right.