Is it possible for a canary diamond to become cracked at the time when its mounting is being replaced from a 6 prong white gold setting to a 4 prong platinum setting. The settings were changed due to the diamond being too loose in the 6 prong setting?

A diamond does not 'crack'. It is the hardest substance in the world. Glass, or even zirconia can crack.

It would take an awful lot to break or shatter a diamond, but it is possible for it to get damaged while being mounted. An extremely thin or very thin girdle (perimeter of the diamond) is prone to chipping.

Diamond Girdle - Learn about Diamonds at Brilliance.com (http://www.brilliance.com/diamond-girdle)