pic_function
Let me see if I understand you completely.
You want to switch from one crystal to another on the fly? I have considered doing this, but some of the complications that came to me are that the impedance of the switch may complicate the signal path in the oscillator, there will be latency while the new crystal begins oscillating, and that anything that needs to be done on schedule in real time will be upset.
As for the name of the subroutine/function (that was your original question, was it not?), how about invoke_Gabriel. It seems appropriate- you're calling upon a power not normally existent within the microcontroller (appealing to an angel) to alter the timebase (play the trumpet at a different tempo).