Hi,

I've had my iPhone 4 for about 6 - 8 months.

Its jailbroken on firmware 4.0.1 with ultrasnow installed (Baseband of 1.59.00)
I've been avoiding updating my firmware in fear of losing my Unlock which would ultimatly make my phone useless.

I would like some information on Updating my firmware to 4.3.1 and having an Unlock still work, hopefully keep my baseband of (1.59.00)
I know there are a few tutorials/guides online, but would still like to be reassured these methods work.

If anyone has any information for me, it would be greatly appreciated.
Thanks

Yes this is possible (you need to be preapred and careful) - I recommend you use Pwnagetool 4.3 (if you have access to a mac)

There are a lot of different guides and many of them are not entirely correct.

What works:
restore with Custom IPSW made with pwnagetool 4.3 to block baseband update
TinyUmbrella - save 4.3.1 SHSH, start TSS server, restore or upgrade to 4.3.1 via iTunes. Then jailbreak with redsn0w 0.9.6rc12

Reported to work (but not supported here)
restore with Custom IPSW made with latest sn0wbreeze (2.5.1) to block baseband update.

I've found these guides online

How to Update to iOS 4.3.1 and Preserve iPhone 4 1.59.00 Baseband [Tutorial]

How to Jailbreak iPhone 4.3.1 with PwnageTool

Also some on iClarified.

I'm familiar with PwnageTool since i used to use that on my Mac back in 2.0 firmware days.

Where exactly is the option to NOT update the baseband???

Thanks for the help

Use the iClarified guides, they aren't perfect but they are some of the best.

There is an option in "expert" mode to "disable" baseband update. If it's greyed out, it means that the baseband will never be updated via this custom IPSW.