Did this recently, great little upgrade. Used white leds from ebay - actually they were the version with 3x SMDs per bulb. Brightened up the clocks nicely.
In an effort to prevent fogging, I smeared fairy liquid onto the inside of the clocks lenses before leaving them to dry for a bit and buffing them up. I also stuck one of those oxygen absorber packet thingies in there. Fogging has not been back since.