Can I Roll Back after Upgrade? [Q]

I’m using an iPhone 3GS running iOS 3.0.1. I never upgraded (Yes, I completely ignored all upgrades in between) because in doing so I would lose my ability to tether, which is quite important to me.
My question is this :
If after upgrading to 4.1 and then jailbreaking, I decide I don’t like it (more specifically I have issues with tethering),can I roll back to the 3.0.1 ? My question presumes that I backed up my iPhone before applying the 4.1 upgrade.
If not, any suggestions on how to upgrade and not lose tethering capabilities ?
Thanks,

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