Gil,

switching clocks with an analog mux is a very bad idea, spikes that violate the minimum cycle time will cause erratic behaviour.

however, there is a safe way to digitally select between two free running clocks, have a look at

http://www.xilinx.com/xcell/xl24/xl24_20.pdf

I've seen this circuit in many applications where the CPU has to be throttled to save power.

Clock A could be direct driven from the oscillator and Clock B from a programmable divider - self explanatory to fall back to Clock A before you mess with the prescaler.

Cheers, Roger