How to create future dated primary worker assignment through HCM REST API call in fusion

We are trying to create a Future dated Primary worker assignment when a current primary assignment exists for the worker through RESI API and we are getting "Primary assignment already exists" error. Tried to add an end-date to current assignment, which is not allowed as well as it is only primary assignment existing.

We need to achieve the following.

When we get a future change in assignment for a user:

:Create a future primary assignment with effective start date as future date

:End date current assignment with future startdate-1

Note: If a Future Primary Worker assignment is created manually, Current assignment is end-dated internally by Fusion