Openhabian makes things much easier. If your using the Pi for other things you can still do it as openhabian run on top of stretch/buster.
You can make a backup using sudo openhab-cli backup --full it will tell you the location it’s stored. Move the backup zip to your PC (I use FileZilla) or USB stick. Then flash the openhabian image to your SD and you should have OH 2.5. You can then restore your backup file and everything should work. You can always test this on a spare SD before moving to main card.
If you decide to go this route let me know and I will provide the steps to restore your backup. It’s not much but I type slow.
Nice post about openhabian and how to set it up. openHABianPi Initial Setup