Can I update iphone 3Gs with new bootrom from 3.1.2 to 3.1.3

[thedogsbollock]

After reading of all the aggro with the Spirit jailbreak for those who already used Blackra1n, I restored my 3Gs to 3.1.2 firmware before doing the Spirit jailbreak

But I'm now wondering if there is any way of upgrading to 3.1.3 without updating the modem baseband, as I'm aware that my iPhone's bootrom is about one number too high for Pwnage to work (so frustrating, does that mean if I had got one sooner I would've had the older bootrom and none of this frustration?)

Any help or advice would be most gratefully received and my sincere apologies if I am going over old ground here, but it would be great to have a definitive answer as the upgrade offer in iTunes is a big wind up

Thanks a million in advance to anyone who takes the trouble to respond
Kind Regards
Bernard

 

[thespazz]

Do you want to maintain an unlock?

 

[thedogsbollock]

Ideally yes.

It's not really so vital now that I have a legitimate Orange SIM, but I am a little loathe to lose the unlock. According to the laws of Sod & Murphy, I know that the moment the phone ends up locked to Orange, something will crop up where I need to use a different SIM