Unjailbreak iPhone 3GS

My wife has an iPhone 3GS that has been jailbroken and unlocked and has an ATT Gophone SIM. I would like to unlock it using the ATT approved unlock procedure. She only uses the phone to make infrequent calls.

Assuming I can unlock the phone using the ATT unlock procedure, can/should I be able to continue to use the current SIM card without loosing the telephone number and Gophone dollar balance?

Are there any issues I should know about?