SugarBPM - opting-out an email address via a BPM process when a contact field is changed.

Just trying to get my head around SugarBPM - and hit a snag. Ive setup a 'processs definition' for the contacts module that fires when a user changes one of the fields in contacts ("Status") from active to non-active.  At that point I want the contact's email address to be changed to either opt-out or invalid but I can't seem to find any way of changing that related field.  

Any ideas what Im doing wrong?