If this previously was a $5 droplet and you resized it using DigitalOcean's flexible resize to the $20 droplet, all you need to do is resize it back down to the $5 droplet. Once your server comes back online from the resize, it will automatically reconnect to ServerPilot. There's nothing more you need to do.
If you didn't start with a $5 droplet and use the flexible resize to the $20 droplet, your best option is to migrate the apps manually to a new server. That is, connect a new server to ServerPilot, create the same apps and databases on the new server through ServerPilot, and then copy over the files and databases from the old server to the new server. Once you're done, make a final snapshot of your original server (just to be safe) and the delete the original server from both DigitalOcean and ServerPilot.
Using rsync will most likely not work. At most, you could do individual rsyncs of each app's web root directory from the old to the new server. You shouldn't attempt to rsync the entire system user or the entire server or else you will likely end up with a lot of breakage now and in the future. It just wouldn't be worth it. Better to do it right this time and migrate manually and then, in the future, be sure to always use the flexible resize if you think you might need to resize back down.