Help! my disc is unreadable

I played Forza 3 part way thru and stopped playing when Forza 4 came out. Recently I decided to play Forza 3 again and further my progress.
When I put the disc in, immediately a message popped up and says an update was available and I let it update. Now the Xbox 360 cannot read the disc after some of the beginning dialog.
I properly cleaned the disc and rebooted the Xbox. The disc was still unreadable. I wanted to play the game so I purchased another copy. This disc is also unreadable.
I am blaming the update. How can I remove that update?
Help me please!