bjneedle you don’t need to do anything in particular to existing notes. Set up your home note and any hub notes you need and add links from your home note to the hub notes (and back links to the home note) and then add links in your hub notes to any relevant existing notes (and back links to the hub notes).
if you have an elaborate folder structure in place you can move toward a tag based collection approach by converting existing folders to smart folders. Notes will be tagged with the old folder name and moved to the top level notes folder. I’m not sure how the tagging works when converting sub-folders so you may want to do some experimenting before doing a mass conversion.
You can add the journaling structure at your leisure.
Good luck!