I have read one report of a brave soul who rewired the 40 mhz clock and a higher
speed clock oscillator's outputs through a double throw switch to allow different
 speeds. There was no mention of any attempts to switch speeds on the fly but
if the SI uses timing loops anything like a Quadra for accessing its floppy
drive, dynamic switching will wreak havoc. A Quadra must reboot after having its
clock speed significantly changed or it will be unable to properly access its
floppy drive.

