Best practice to terminate implementation employees

As many costumers, we did our implementation of HRMS after Financial Implementation. Now we have several employees that were created only for implementation purposes. I we need to terminate this employees because are contaminating our reports.

We have two year that we end our HRMS and payroll implementation, but my predecesors did not worked in this issue.

Is there a document of best practice to do this?

Our preocupation it is related that how can we minimize our impact with others modules when we end these employees if they still using them?.

Making a full test in all modules related it is the only way?