The net result is that all of the cash shop methods of acquiring the current crop of dragons will be retired on April 20th, which is (not coincidentally) when the new round of dragons will be introduced. You will still be able to pick up the old dragons in the Dragon Scale or Dragon Token shop, but that will be the only way. So it’s out with the old, in with the new, and if you really want those existing dragons, you should work on that before next Thursday.
Source: Official Site