Change primary assignment

Summary:

We have an employee where we want to change the primary assignment to secondary.

Content (please ensure you mask any confidential information):

In the employment screen, the assignment has been changed to secondary, and another assignment made primary.

However, we are unable to delete the assignment that was previously the primary one.

Is there somewhere else that the change needs to be made, or any other steps that have been missed?

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):