Go to saved data utility
Go to GT5 and open the folder
Press triangle on the save file
Click copy and it brings up all the places you can copy it to
The second one down should be the memory stick
Select that and it should be backed up
Then when you want to replace the saved data:
Saved data utility
Go to memory stick, second one down again
Open and Gt5 saved data is there
Triangle
Copy
Replace existing
You should then have the game file back as it was with the gold chrome paint chip still there....... repeat....... repeat...... and so on
Edit:
Make sure you send it to the secondary account and only do this copying and replacing on the second account, best way to avoid any accidents with overwritting the save file of your main account..... don't want to see any heads being banged off of walls here