Solution Request for Giving Existing Persons The 'Contract'

Dears,

We trust this finds you well.

Kindly be aware that the instance had originally an [Employment Model] that didn't include the 'Contract', as it was "2 Tier - Multiple Assignment" previously, and then it was needed to change it to become "2 Tier - Single Contract - Single Assignment" so we can have the 'Contract'

How can we know the steps required to affect & impact the already existing persons, and not only the new ones?

Later on dears, kindly note that we also tried achieving this via HDL as attached, but we weren't met with success unfortunately!
[KINDLY FIND THE ATTACHED HDL THAT WAS UPLOADED TO THE INSTANCE]