3G - unlocked and jailbroken on 3.1.2 - upgrade to os 4
I apologise if this has already been discussed to death but i'd rather as a stupid question again before taking the plunge. If I loose my unlock i'd be screwed! So please bear with me.
Details of the iPhone as is today.
1. IPhone 3G 8GB
2. Dont know whether its an early or late model boot rom
3. OS: 3.1.2 Jailbroken & Unlocked
4. Dont know how it was unlocked / jailbroken. Blackra1n was installed so I guess it was using Blackrain.
5. SHSH File Not backed up but i recently clicked "Make my life easier" for the 4.0 OS in Cydia.
6. I do not have an original carrier sim so it would need hactivation also
Would like to upgrade to 4.0.1 running on a windows machine and need to preserve both my unlock & jailbreak. I cannot afford to loose my unlock at any cost as it would make my phone useless/
Can you please advise me on the steps & risks. Should I us the default itunes update?