I think this depends on how much you want to adapt to the new MainUI. I wanted to move most of my items into the database, so I’ve been importing them. If you want to do that, you just have to discover the things, then take your time importing your various items files until they’re all set. After that, you can just copy your rules over and you’re good to go.
But if you just want to get it working as quickly as possible, use the backup method suggested by James. I just wanted to treat it more as if I was starting from scratch.
You should just be able to unplug the controller and plug it into your RPi. However, if you have any devices that have been securely included (such as door locks), you need to copy the security key from OH2 BEFORE you add those secure devices in OH3. I made that mistake, and it proved a bit tricky to recover (though it should have been easy). I suggest reading this thread.
I suspect that the openhab-cli backup will copy over your security key, but I’m not 100% certain of that.