what process to upgrade my 3gs?

[MrDove]

I have an Iphone 3gs currently running 3.1.2. Baseband 05.11.07 I did my original jailbreak and unlock using Blackra1n.

I've been wanting to upgrade for a while but I've been afraid to lose my jailbreak and my unlock since I'm using T-Mobile and messing up my unlock would mean I would lose all my phone service.

What steps should I take to ensure that I can restore in the event that I have a problem? What program/procedure would I use for this phone?

I know that a few programs are Mac only. I'm running a PC with Windows 7 if that matters.

 

[Applejuiced]

Open up cydia and look at the top of the page if you have SHSH's saved so you can restore to 4.0.1 and then JB and unlock again.

 

[MrDove]

Okay, I restored as instructed but now I have the "unsupported sim card" error in itunes and therefore can't access jailbreakme.com

How is the unsupported sim card error overcome?

 

[TMar]

Okay, I restored as instructed but now I have the "unsupported sim card" error in itunes and therefore can't access jailbreakme.com

How is the unsupported sim card error overcome?

You either need an AT&T SIM (doesn't have to be activated) or you should have stayed on the train.

EDIT: Isn't this one of those times where it would have been easier just to recommend the custom ipsw's?

 

[Applejuiced]

Sounds like you updated it already. Good luck now.
You might be stuck with 4.0.2 or 4.1
Both that cannot be jb or unlocked.