Switch Primary Sales Rep on a Customer while leaving sales team intact

We have several thousand accounts having a sales team with three sales reps and two other sales roles. I need to change one of the sales reps to primary and 100%, the current sales rep should be changed to 0%, not primary, leaving the rest of the team intact. I came across a solution that uses the "replace sublists" option via a csv upload, which works when there is only one salesperson on the customer sales team. However, this approach does not work in our situation, and I cannot find a way to make it applicable here. Any suggestions? I