I don't think you can do that... I remember going to PSN Store looking for the game and everything I ever paid for its there to download for free.
I also remember the LE cars in GT5 appearing auto-magically upon booting the game after having the game data deleted to win the penniless trophy after which I restored the old data (i obviously had the backup... so much time cannot be wasted

)
Now that I think about it, it should be the same thing as it happened in GT5. If you have a
backup maybe you can try this:
- uninstall patch
- delete game data file
- boot the game (probably you'll have to do those 1st races)
- ignore patch warning
- check if everything its there
- exit/load the game
- install patch
- exit game
- restore game data file
- load the game
Just following the same logic. I haven't tried because I haven't lost LE. Bless my PS3

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