I'm in the same boat, except moving from 4GB to 16GB flash drive - now that initial-release-bugs have been more or less worked out, its time to move up to 9.3. But first I need a larger boot drive.
When you say "install os on new USB", this is done as if installing FreeNAS on a new xxGB USB flash drive for a new system, without regard for existing drives and jails and all the rest - right? Then importing the saved config file modifies the new boot drive so it has the same setup as the original (sees and can access the volumes, the jails, the scheduled scrubs and cron'd rsync backups and all the rest). Just clarifying my understanding before I bork something - I have a nicely working system with ZFS drives serving my media just the way I like it. I'd like to minimize post-resizing surprises going through this exercise.
Thanks SnL for the concise answer, and to doglover for asking my exact question!