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

after reading of aggro spirit jailbreak used blackra1n, restored 3gs 3.1.2 firmware before doing spirit jailbreak

i'm wondering if there way of upgrading 3.1.3 without updating modem baseband, i'm aware iphone's bootrom 1 number high pwnage work (so frustrating, mean if had got 1 sooner would've had older bootrom , none of frustration?)

or advice gratefully received , sincere apologies if going on old ground here, great have definitive answer upgrade offer in itunes big wind up

million in advance takes trouble respond
kind regards
bernard

 

do want maintain unlock?

 

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