I'd do a scrub of your pool and export it on version 7 before you upgrade. Then it should be as simple as flashing a new USB disk, plugging it in, booting and doing your basic config (network etc.), and then going to Storage->Volumes->Auto Import.
You could try it now and go back to version 7 as long as you don't do a "zpool upgrade" to your pool until after you're ready to completely switch over. It's probably a good idea to export your pool each time before switching between 7 and 8 while you're experimenting.