How Detect Field Changed Old and New Value using form and field rule

How Detect Field Changed Old and New Value using form and field rule

hi there

i want to disable technician selection when technicians change site from SiteA to SiteB , i tried to do it using form and field rule but i do not know how can i get old value of site field before changing to new value, because i want to look at old and new value of selected site to disable technician selection 
i need something like this:
on Site field changed {
 if (oldSiteValue=='SiteA' and newSiteValue=='SiteB') {
  Disable Technician Field
 }
}

regards,
omid