iPhone 3G - Downgrade 4.0 > 3.1.3 preserve baseband?

[Xero009]

Hey, I've been reading 'til my eyes hurt about upgrading and downgrading, but I can't seem to find the answer to this particular question.

I've got iPhone 3G, version: 4.0(8A293) modem firmware: 5.13.04

It's unlocked for T-Mobile.

I want to downgrade to 3.1.3 (for speed reasons)

It looks like I simply make custom 3.1.3 firmware with sn0wbreeze, shift restore with iTunes & all is well.

However I'm not sure that doing this will preserve my modem firmware? I really can't screw this up

Thanks!

[z6joker9]

Well custom firmwares take off baseband updates. However, you're downgrading, so I don't think it matters anyway. Why would you care if the baseband gets downgraded? 05.12.01 (3.1.3) is unlocked through ultrasn0w the same way your current baseband is.