No...
That wont work
Ok heres why... The 2.31 SPL is NOT overwritable so that means when you flash a ROM it wont over write the SPL so you dont have to keep unlocking every time you want to flash a custom ROM(kind of nice) ok the bad part.. That means it cant be unlocked... Soo... the only way to unlock is to downgrade the SPL to 0.40 which IS overwritable and then flash a NON gps ROM... Which will overwrite the SPL but there are dangers in doing this.. Then you just flash back to the 3.03 ROM from Sprint..
Here this thread might help
http://forum.ppcgeeks.com/showthread.php?t=26082
PLEASE do not go off what im saying read up first.. I might be incorrect and I dont want you to brick your phone if I am
Lets see what other people have to say also...