Changing employment start date for non-primary assignment within the same work relationship

We have an employee with one work relationship and two assignments: one primary and one non-primary. The non-primary assignment start date is incorrect and needs to be changed without affecting the primary assignment or the work relationship start date.

We can see the Change Assignment action, but it does not expose a start date field in our environment.

Is there a supported way to correct only the non-primary assignment start date in this scenario, or would this require correcting/recreating the assignment through another process?