If you have not edited in XML, I would suggest that assignment of notes is NOT a good place to start.
It is a more complicated thing than just making a single change somewhere in one XML file.
If you look at your My Paratext 8 (or9) projects file listing, you will see that there are Notes files for every team member, and those notes files contain the relationship between the note creator, the assigned person, any other people who have chimed in on a note, so what we think of as one note has tendrils in all of those files.
If no expert chimes in on this thread to say “here’s how to change these 5 notes at once” then I suggest you make it as a suggestion using Give Feedback, Make a Suggestion about Paratext in Paratext from the Main Menu. Many times when I have made a suggestion, the expert who answers my suggestion by email gives me hints on how to solve my problem.