I have followed this guide until step 9 to create a jailbroken 3.1 .ipsw file, which doesn't update the baseband and allows ultrasn0w to work, on a friends mac with pwnagetool.

Can I now update my 3.0 3GS phone on my windows computer using that .ipsw file?

I need to unlock it so ultrasn0w has to work, but as I understand it it will?