Dragon metamorphosis

I enjoy the opportunities opened by the concept of monster evolution, as much or even more than that opened by spontaneous generation. Curiously, there are already number of stories from around the world in which an animal or a person becomes a dragon.

• The dwarf Fafnir became a dragon as a result of the curse on a treasure hoard.
• A Chinese story recounts that a koi fish spent a century trying to jump a waterfall: when it succeeded it ascended into a dragon, and thus the waterfall was named the Dragon's Gate.
• A Korean story recounts that a serpent found a fallen star, which granted his wish to become a dragon.
• Chinese myth says that pearls are tears of the Moon, and some were taken by dragons. One Chinese story tells of a boy who found a magic pearl that could produce endless rice. When he accidentally consumed it, he became a dragon.
• According to Serbian legend, several animals including carp, snake, ram, and rooster could metamorphose into dragons when they reached a certain age.
• In more modern fantasy fiction like Elizabeth A. Lynn's Dragon's Winter or the Dragon's Gate MUD, dragons are born resembling human beings and become recognizable as dragons as they mature.

On a related note, D&D and derivatives often depict dragons as having age categories. The older the dragon, the higher its CR. Dragons that don't have age categories are screwed out of luck. With monster evolution, that limitation is gone. Now the lowly wyvern can evolve into a higher CR dragon, and high CR dragons may be weakened and reverted to lower CR dragons.

I can't wait to do more thinking on this subject!