Hey there, ving! All you have to do is load up GT3 without any memory card. The PS2 will give you a prompt saying you don't have one in there, but that's "OK".
The Simulation Mode will start on Day 0 (no more license tests) and 0% complete, and the Arcade Mode wil have only the 6 A-area tracks available. Time Trials (and Free Run times) will also be "zeroed".
If you were looking to "replace" your gamesave with a Day 0 game, just re-insert your memory card, and save the Day 0 game to it.
If you were looking to actually delete the GT3 gamesave, you can do this too... If you put in your memory card and go to the Save Game screen (where "SAVE GAME" is in red, and it shows the 2 orange memory cards), you can press the "Start" button to enter the File Operation Menu (the scrolling text at the bottom says so). You can copy/delete/rename anything you've saved to this card, including the GT3 Sim/Arcade Mode gamesave.
As far as the licenses go, there's no way to get them back (after deleting/overwriting the gamesave) other than thumbwork and/or a Shark.
I hope this helped! Have a great drive, ving!
Hazzard
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