Here's a step by step walk thru.
First, save any photos or replays to "XMB" if you want to keep them
Make a back up of your game file, just to be safe.
1. Start GT5
2. Borrow any car from a friend
3. Quit GT5
4. Select from XMB "Game Data Utility" (6 spaces up)
5. Press "triangle" on GT5 file to open options
6. Select "delete"
7. Select "yes" and wait for ps3 to do its thing
8. Press "O" to return to XMB
9. Start GT5
10. Deny "latest update" by pressing "O" (u will see "creating system files" screen & game start-up vid)
11. Press "X" to start game
12. Select GT mode
13. Open your garage
14. Select any other car in your inventory
15. Quit GT5 again
16. Restart GT5 again
17. Install "latest updates" and wait for all 6 downloads. (if you want other cars, go back to step 1 after this)
18. Load all other GT5 data as prompted
19. You're done
If you want to get more than one car from your friends, skip step 18 and return to step two.
If you follow this guide step by step, all you will loose is custom tracks (way around this one, time consuming) & some thumbnails (easy fix).
I also lost a great car in the deal, the "EDGE" special RIP
You may loose everything but what you had borrowed.
Backup, backup, backup.