In Sakari, you can allocate a Sender to a single user using a single-user Group.  Assuming you have done this, you can change the Sender allocated to a single user by following the steps below, depending on your scenario:

Sender is new

1. Create a Group for the new Sender
2. Name the Group to match the user's profile name
3. Add the user's profile to the Group
4. Delete the previous Group (if one exists)

Sender already exists in a Group

1. Add the user's log-in profile to the Group
2. Rename the Group to match the user's profile name (if needed)

Transferring Sender to another user

1. Remove the current user's profile from the Group that contains the Sender
2. Add the new user's profile to the Group
3. Rename the Group to match the new user's profile name (if needed)

Previous Sender is no longer needed

1. Remove the Sender from Sakari via the Senders section (Remove a Sender).  This automatically removes the Sender from its Group.
2. Add the new/replacement Sender to the same Group.