Q:

 Is it possible to change clocks run-time? 

A:

   Yes, the source for the high frequency clock can be changed run-time by writing to HFCLKSEL in CMU_CMD, while the low frequency clock source can be changed in the LFA and LFB fields in CMU_LFCLKSEL. Remember to make sure that the clock source you want to switch to is enabled and ready before you change the clock source.