just sync the new device with your sync-target (dropbox). have your password for encryption in reaching distance.
there's no rocket science in it
and of course the general advice is to use the simple backup plugin before or export all notes manually before. but from your post, you are on the safe side here already.
IMHO the question is valid nonetheless. 'Migration to new PC' can be speeded up a great deal by copying over the profile folder first, but I wonder if there's any risk in this. From my experience with half a dozen instances of Joplin Portable: there isn't. But I'd be interested in what others have to say about this.