Red error pop-up: Failed to save attendee <name>. Error message: User is already used.
For Attendees already in the system:
step 1. Go to Attendees > Check via search if the email is already in use for another attendee within your event. If not then this user is known for another event within NetworkTables.
step 2. Go to Attendees > Add this person as new user.
step 3. Update the information to this user.
For Session Hosts already in the system:
step 1. & step 2. same steps as above.
step 3. Go to Sessions > update the host to the new user. Read here how to change a session host.