It is not hard to do and I did a similar thing last week. You will need to download a ipsw file and then go into recovery DFU, which you should know how to do if you have jailbroken and unlocked the phone before. Reinstall 3.0 and then unlock and jailbreak.

http://www.ismashphone.com/2008/08/recovery-dfu.html

http://www.iclarified.com/entry/index.php?enid=4302

http://www.iclarified.com/entry/index.php?enid=750

I then upgraded to 3.1.2.... its easy and it worked. I downloaded the custom ipsw files for the iPhone 3.1.2 upgrade from this website. This person has all upgraded custom ipsw files for all iPhone models.

http://www.sinfuliphone.com/showthread.php?t=3306

but I did so after taking advice from the people who developed the unlock for the iPhone from their blog.

http://blog.iphone-dev.org/

My iphone is unlocked and on the current version of software.