I have these shsh on cydia: 3.1.3, 4.0 (not sure if it's real coz I did tiny umbrella after people said Apple stopped signing), 4.0.1

I have 3Gs currently on 3.1.2 with old baseband 05.11.07, non-tethered with blackra1n. I am comtemplating on upgrading to iOS4, but I want the option to go back to iOS3. Now since i don't have the 3.1.2 shsh with cydia, I would assume 3.1.3 is my only option.

My question is, am I able to upgrade to 3.1.3 and be jb/unlocked at this point? Would this (How to Jailbreak/Unlock 3.1.3 iPhone 3GS/3G/2G iPod Touch 2G/1G with PwnageTool OS X) be the guide I should follow to upgrade if i'm already jb/unlocked?
If that's the case, it would mean I can actually restore my phone to a stock firmware 3.1.3 (no jb/unlock) for ie. phone repair.

thanks in advance for your input