I have a Live DVD of Ubuntu 13.04, and a 10GB persistent ext2 casper-rw partition on a 64GB USB stick. What I want to know is, when Ubuntu 13.10 comes out, will it be possible to use the new DVD image with the old casper-rw partition, or will I have to reformat and reinstall all my applications?
I will try to answer as best as I can. I have never tried this, so I can't be sure if the following is correct.
If the casper-rw partition contains system updates (kernel, updated software, etc.) surely that won't go well with the new DVD.
Particularly, some software are updated only between versions, such as LibreOffice. So any patches to older version of LibreOffice meant for 13.04 won't work for the Newer version that will come with 13.10.
Your saved personal configurations may also be off, if the format of the config file has changed between versions.
Make a copy of all your personal data and keep your fingers crossed. If things go south (if it does not work) start again with a new casper-rw.
Consider a full install in the USB drive. Assuming your computer can boot directly from the USB drive. See What would be the differences between a persistent USB Live Session and a installed Ubuntu in a USB drive? for advantages off a full install.
Hope this helps