Got a question for a jailbreaker who's bought a second hand 3GS or 4 before.

I'm supposed to pick up a preowned iPhone 4 (with 4.1 FW) this afternoon. I currently have a 3GS, and want to switch over to the iPhone 4, but I can't let the firmware update to 4.2.1.

I plan on cutting my existing sim card down to fit. The videos online look super easy.

If I backup my 3GS and "restore from backup" will it update the firmware?

Or do I set it up as a new phone, and then just insert my sim? I assume setting it up as a new phone won't update the FW...

I have over 100 apps all organized in folders. If I could restore from backup, it would save me a lot of aggravation reorganizing everything...

I'm not sure if the new phone's blobs are saved w/ Cydia. I use Tiny Umbrella to save the blobs on my 3GS. I suppose I can plug the new phone in and quickly save them, but I've never needed to use them, so I don't know the exact process w/ Tiny Umbrella.