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