This. You can downgrade your FW at will but you can't with your baseband. It would suck if you tried to update to a custom IPSW through pwnage tool and have something go wrong to where you are forced to restore thus updating your BB and losing your unlock.
Seriously, don't.
You won't be able to use ultrasn0w... and blacksn0w (blackra1n is the JB tool) is much less stable.
If you are desperate to have 3.1.3 then miss out stage 1. and keep your 4.26 BB