I'm using a kubuntu liveUSB with persistence. It's possible to update the liveUSB itself, such that the updates persist. However, the update packages aren't used for installation, as far as I can tell. Is there any way of updating the packages on the install image, such that on the next install, those packages don't need to be re-downloaded?
I'd just try with an updated copy of the LiveUSB (keep the one that works in case anything breaks), the persistent files should all be user data files (no changes in the LiveUSB to those) or local configuration (unless the distribution has changed radically, they should continue working just the same). Check which files are newer than the ones on the USB stick (
Oh, and please answer your own question if it works/breaks, with the exact versions you used.