It's easy to do (if your careful and know how not to burn yourself of your toy with a soldering iron, and the effect can be quite stunning.
Whats even better, is that the latest LED's are so much brighter and clearer than their parents so look great when the light and turned off and everything it flashing.
The flashing rate of these LEDs is slow but impressive.
Here is a remote control I changed using this method.
And here is a link to eBay where you can find them, as with some many commercial flashing chords etc taking priority list, the LEDs are hard to track down:
#fixed1tTOYREPAIRSHED