CRM 2015 setLabel not working for field in Business Process Stage

Hello,

I am trying to change the text (label) of a field that is in a stage of a business process flow, but it's not working. I am using the following JavaScript code:

if (Xrm.Page.getControl("header_process_testfield") != null){
    alert(Xrm.Page.getControl("header_process_testfield").getLabel());
    Xrm.Page.getControl("header_process_testfield").setLabel("new label");
}

The getLabel works fine and returns the current label/text of the field, however, the setLabel does nothing. There are no error messages or anything. If I use the setLabel method on a field located on my form it works fine, therefore I'm assuming the way to do it with a field found in a Business Process Stage is different.

Any assistance you can provide will be greatly appreciated.

Thank you in advance!