next Unlockable update from 3.1.2 on 3GS?

[The1TexasRanger]

Hi all,
Got a 3GS on 3.1.2 5.11.07 BB Need to update and keep unlocked. What is the easiest safest way and which firmware? Ive been told I need to make a custom firmware package to keep old BB? I will be using windows to jailbreak.
Thx.

 

[Applejuiced]

That would be 4.0.2
Or use pwnage to save your baseband.