This changes part of the character01 file. Delete the extracted file if you want things back to normal.

Unverified. Go to the file character01.rar in the directory .../install/gtasa/... and extract it. You will find the code for the character known in the game as C.J. But if you want good old Vercetti back, then change the three digit number from 0 1 1 to 1 0 0. This should change the character, I don't know if this will work on all of the people's computers but it worked on mine.