I’d start with a fresh new SD Card and use openHABian, which provides already a openHAB3 (milestone or build) option with sudo openhabian-config.
then do as described as above.
and then here
I myself switch from textfile-based configuration (OH2) to UI-based configuration (OH3) and therefore don’t migrate something, but built anew from scratch, also to get rid of some old f$%ked up namespacing of items and stuff and I move to the new Model-based approach, which I really like…
so basically I already started a few weeks back with the milestones and learned by doing. OK, I do have a whole bunch of Pis to do my tests, and my “old” OH2 still runs as main instance and will until I finally pull the switch to OH3…