Is there a way I can try iocell while I've got existing iocage jails?

'It depends' ;) First of all, if you're using 'stable iocage' then you're out of luck, since too much changed in devel iocage, and iocell is based on devel branch. Second, it would either involve renaming your datasets in few places (from iocage to iocell) or changing few strings in iocell (from iocell to iocage). Other than that, iocell aims to work exactly the same way as iocage, with same principles, workflow and so on. Check out the commits history in iocell `develop` branch to find the one where iocage name is being changed, it should give you an idea on what and where to do. I am also planning some migration guide for existing iocage installations, but it might not be very easy/very stable, I am afraid.

Okay, let me ask it a different way then.

If I had to move away from iocage right away, how would I preserve my jail? I don't really care what the jail is called - I just want Plex or whatever to start up when I start the jail, and to be able to keep upgrading packages inside it.

That's entirely different question, and doesn't really have much in common with iocage/iocell. I, for example, have every single thing I host automated in a way where it doesn't matter wether it runs in jail provided by iocage, jail manually built, vm in AWS or a physical machine. This way, my migration path would be: wipe existing jails, create new ones with new tool, launch automation, done. You, however, might need entirely different approach, based on how your environment/setup looks like right now. One very primitive way would be to simply archive (rsync, tar, whatever else) jail contents and deploy it in jails created with new tool.