I have a 3Gs (3.12) jailbroken with Blackra1n. I have lots of apps installed (Appstore and Cydia) and everything works well and is pretty trouble free. I tried iOS4 shortly after it was jailbroken and I had an issue where every time I tried to reply to an email, the mail app would crash. So I went back to 3.12 (using my SHSH blobs) and I've been there ever since.
But now the Netflix app is out and I *really* want to install & use it -- but it won't let me. Says I need 3.13.
I really don't want to reinstall everything. Is there a way to upgrade a jailbroken 3Gs from 3.12 to 3.13 (jailbroken) while keeping all my installed apps & data?
There are apps in cydia like aptbackup and pkgbackup that can do that for you. I dont usually recommend them for updating but for a small update from 3.1.2 - 3.1.3 you should be fine using one or the other. You should probably do a little research to see which one best suits your needs first though.