HOW TO PRESERVE CURRENT BASEBAND OF YOUR IPHONE
If you have unlocked version of iphone and trying to upgrade its ios to a new one then think twice,because it might be very easy to upgrade your iphone to a newer firmware but you may loss the ability to unlock your idevice. So, actually what happens is that whenever you upgrade your idevice firmware the baseband of your idevice also gets changed or upgraded but the ability to unlock your iphone completely depends on your baseband version. Therefore i have presented this tips for you to get rid off the change of your baseband while upgrading your iphone firmware.
This tutorial works for idevice from ios 3.x to 5.x.
Extract Snowbreeze to a folder.
Note: If you have not downloaded the original ios firmware then select “iOS Firmwares are downloadable here ” optiion in Snowbreeze window. Then select you idevice and desire ios firmware to download. Then goto STEP-5
Now just put your iphone in DFU mode and restore from itune.
Now you can easily jailbreak and unlock you iphone.