What do I save to ensure successful downgrade to iOS 4.1 on iPhone 3G? I'll do the upgrade from ios 4.1 jailbreaked (on an iphone 3g with a foreign locked sim) without updating the baseband.
My baseband is 05.14.02 (bootloader 5.9).
So I should to obtain an iphone 3g ios 4.2.1 jailbreaked with the 05.14.02 baseband (I'm not interested to use it for phone calls).

I would to know what I have to save before the upgrade to revert to the original state (ios 4.1 jailbreaked) in case of failure.
You need to save the SHSH blobs for your phone.

This can be done by:


Allowing Cydia to save them for you automatically.
Using TinyUmbrella to back up the blobs to your computer.
Install iSHSHit from Cydia, which can access your SHSH blobs, save them on your phone, and send them as email attachments.


Currently Apple is blocking access from Cydia to the servers that give the SHSH blobs out, so options #2 and #3 are left.

Finally, Apple is no longer signing for 4.1, so unless you already backed up SHSH blobs somehow, you will not be able to downgrade.