I bought my Iphone 3gs from AT&T and jail break it. My version is 3.1.2 I want to upgrade the version but some people are saying if I do that than my Base Band would be completely broken and there is a 95% chance that the phone would not work at all. I want suggestion from the experts.
They are wrong, the baseband can be preserved or if the phone is old enough, can be updated to iPad baseband and then downgraded back to a unlockable baseband that has functional GPS(iPad BB allows unlock, but breaks GPS). If you don't want to unlock using jailbreak tools, then by all means update, it will not hurt anything and chances are ATT will give you a free unlock if you have completed a two year contract with the phone(call them or use the web chat to ask for a unlock, you provide the phone's IMEI and wait a few days for the email). If you want to be able to unlock using jailbreak tools, you will want to either preserve the BB(I assume you have 05.11.07) using a NO_BB custom IPSW made with Redsn0w or Sn0wbreeze to update with or just update with stock, then if your phone is older than week 28 in 2011 you can flash the iPad BB followed by downgrade from iPad BB to 05.13.04, then jailbreak and install ultrasn0w to unlock.
Only thing to worry about is, if the phone is newer than week 28 of 2011 and you are trying to flash the iPad baseband, it will brick the phone.