She needs to call the 1K line or another reservation line. This requires more experienced agents and a supervisor in the end to make it happen.

If she is already at the airport and taking the outbound flight - this will not get any easier. Best is to wait till the outbound in complete and then call in to make the change. Generally not recommended to doing this mid-itinerary, but it 'should work'.

Good luck. Btw, the reservation number does not change - the flight number will change

Also, make sure the booking class is eligible for +P upgrades