As I said above, it's definitely an Epic Destiny (though I could see a ritual used to "retrain" to said epic destiny -- or just have it be a story approach). I mean, to make a dragon, you need: a "breath" weapon, flight, larger than normal size and reach, and probably claw and/or bite attacks. Those aren't things you get out of a ritual -- or even out of a single power; those are class features.
To make a new one...base it on an existing ED and tweak as necessary. You'll notice, if you're honest with yourself, that you can't really justify all of that with one epic destiny--probably, so if you really want to be a dragon all the time (rather than, say, a silver dragon who is in human form most of the time and only takes dragon shape for emergencies), you might need to use a custom paragon path, a custom epic destiny -- even a custom multiclass tree to house all the bits that make up "I'm a dragon".
Actually, there -is- an epic destiny already that lets you turn into a dragon -- albeit only twice per day -- Look up Draconic Incarnation in Dragon 388! If you want the easy path--you're a silver dragon, but for your own reasons choose not to take draconic form except when it's urgent -- thus when you "die" you switch to your dragon form and finish the battle that way (working with your GM you might want to trade out insubstantial/phasing on the Dragon Spirit 24th level "once per day, when you die become a dragon and finish the fight that way" feature for something close to the 30th level form), and if you're 30th level you can once per day choose that a fight is important enough to turn into a dragon without "dying" first, gaining huge size, flight, reach, and an at-will breath weapon. Obviously, swap fire for cold, but that's a trivial swap.
