Many years ago I was on a SAR flt in the UK. Our aircraft would operate on training sorties as SeaKing 190 and Seaking 191. As sonn as they were tasked by RCC the callsign would change to Rescue 190 or Rescue 191. This could happen prior to start up if scrambled, or mid sortie if tasked whilst airborne. It was the aviation equivalent of switching on the "Blues and Twos" I guess you could say. Suspect that the situation referred to was similar. Patient condition deteriorated perhaps, or the aircraft was positioning and was then responding to a call received whilst airborne?