If you want to go through all the patches again, then it is do-able. I did it a few months ago to play around with some cars at the 2.0 patch.
I saved my GT5 game save to a flashdrive at the then current updated patch first.
I then deleted the GT5 game data and started up GT5. I used my back-up game save data, but some of the cars were blank. These were the cars that did not exist at the time such as DLC cars. I then installed all the updates up to 2.0. I simply canceled after the 2.0 was installed. I did this because there were a few cars that you could paint the stock wheels on after the 2.0 patch. I played around with several of these cars, painting the stock wheels in various combinations. I even learned how to paint the stock wheels and change to another car while still leaving the car in 0/0/0 form.
If you decide to downgrade, you will lose all your pictures from within the game, and you will lose all your tracks you created. You can transfer the pictures to your PS3 picture files, but I don't think you can transfer them back into the game. Don't know much about the track creation; only created one track that I lost.
You will not be able to play on-line at all unless you have the latest update installed.
I did not lose any DLC, and the DLC cars I had used were re-installed fresh and new with 0/0/0 stats. Some of the DLC I did have to re-install from the PS Store.
I ran into a problem, however, after I installed to the latest update. Most of the cars with painted stock wheels did not save correctly. They were listed in my garage, but there was a error message saying it had to be re-downloaded (or something like that). I essentially did a lot of cars for nothing, only a few of the first ones I did stayed intact with painted stock wheels.
There is also another problem I've noticed, and I think it has something to do with me downgrading GT5. My PS3 will not show the latest members online that I've met. It has a list of people I've met, but they all have a date of 12/14/2012 (yeah, in the future), and when I go on-line, the list does not change. I haven't been able to find out how to get this corrected, and I think there was been a PS3 update since. Oh, I do play MW3, and all the people listed were from MW3, and I think the latest players met list is screwed up there as well, except for the last two slots. I haven't tried re-installing MW3 to see if this corrects it or not, so I might do that soon; just need to back-up MW3 again.
If you do decide to do this, and you decide to play around with some of the cars at the 2.0 patch with painted stock wheels, let me know if it works for you. Maybe we can trade for some of the cars. I have several Maserati Gran Turismo S w/ painted stock wheels for trade. There were a few Maserati's I lost that looked cool; the burgundy color with stock burgundy painted wheels, white w/ white painted wheels. I do have a blue with stock blue painted wheels in my garage (used the same paint chip the car came with for the wheels), but I think it has miles on it. One of the cars I was really hoping to keep (I actually downgraded GT5 twice to try this) was the High End Performance G37. I had several variations of that car with stock painted wheels, and they all failed to stay in the garage. I had the stock wheels painted from various colors of the car with chrome, metallic grey, matte black, red (with the red car), and blue (tried to match the blue from the rollcage inside the car). Hell, I might try this again just to see if I can get that G37 to work.
. Perhaps some photos of standard cars then and now, and some comments on the physics changes.
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