The BAGTAG is not restricted to one user. However, for security reasons, the moment a BAGTAG is updated, it will be "locked." This lock is on the BAGTAG along with the email and name of the passenger. The lock is active from 48 hours before departure time and 24 hours after departure time and is only set if the flight is in the future.
The user receives an email (confirmation message) with details about the update and an unlock link.
Within the lock period, someone with the same email and passenger name can simply update the BAGTAG.
If the BAGTAG is used during this period with a different email address and/or passenger name, the update will be blocked.