What is the best option for upgrading my iTouch 3g to iOS 4.0 while keeping my jailbreak?

I currently have an iTouch 3G that is jailbroken and running 3.1.3. I want to upgrade it to 4.0, but with the iTouch still jailbroken. I know I can use jailbreakme.com to jailbreak 4.0, but I’m not able to upgrade at the same time. How can I move to 4.0/Jailbroken easiest? Will I have to upgrade to 4.0, lose my jailbreak, and then re-jailbreak it?

Thanks,

Cameron