A sales rep has left the company and an administrator has been asked to re-assign...

The mass transfer tool allows you to transfer up to 250 records at a time from one user to another user while keeping the existing team members intact. You can access this tool from Setup by entering Mass Transfer Records in the Quick Find box. References: https://help.salesforce.com/s/articleView?id=sf.mass_transfer_overview.htm &type=5