Your question makes no sense. You will have to upgrade to 2.1, then re-jailbreak. Never used AptBackup, but others in the forum have had success copying files off the iPhone via SSH, restoring, and copying the files back.
AptBackup is simple to use. Just install it. Open it up and click Backup. When you sync your phone. The list will be saved. After you jailbreak 2.1, install Aptbackup, open it up and click on restore. It will bring up a message saying it's restoring all the packages you've downloaded from Cydia. the phone reboots and you're good to go.
I've used it a few times now and have yet to come across a problem.
*Note (Customize is broken in 2.1. So any file sets you downloaded will not work until it's updated.)
No problem. The thing that shocked me was, all the themes (battey, keyboard, etc) I downloaded for Winterboard were all in and already selected. So after the phone rebooted, everything looked like it did before.