You must rename all the interlagos2001 files within the wad, including any interlagos2001 files in sub folders. You have to rename the interlagos2001 subfolder itself too. But do
NOT rename any tex files!
So just rename the .cpf, .col, .env, .gp4, .hze, .vcl, and .crd files plus the 4 .bin files. That's 11 files (plus one subfolder) to rename.
Then, like you say, alter the prefix with cmagic.
Once that's done, there's one last thing to do or it won't work. You need to add a bahrain.anm to your anim.wad file. Just copy the interlagos2001.anm in your anim wad and rename it as bahrain.anm (or whatever your prefix in cmagic is). Keep the interlagos2001.anm, as it's needed for interlagos!
There's other threads around with this info in it too. You can also add the 'country' the number of 'laps' and the tracks 'name' to cmagic also. This makes the tracks slot completely independant. You can put it anywhere you like in GPxPatch.
Using this method you can have tracks for any year you like named (for example) interlagos2001, interlagos2005 etc etc and run them all in the game side by side, and in any slot you wish.
Hope this helps.
Regards,
Mr Pibb.