I just read the Korean folk tale "The Magpie and the Bell" (I don't know whose translation this is), in which a huge snake claims to be able to transform into a dragon and rise to heaven:
"Fine. Listen carefully. Deep in this mountain, there is an old abandoned temple. No one lives there now. In that temple, there is a huge bell still hanging. If you toll that bell three times, I will let you go." "Yes, that can't be too difficult." "But wait, my boy! You are jumping the gun! Don't be so sure! You must ring the bell from here, from right where you are now!" And the snake continued to tell him that if he tolled the bell three times she would be transformed into a dragon and rise to Heaven, and that that was the only way he could save his life.
How is it possible for a snake to transform into a dragon? Is it a standard trope/belief in Korean folklore that snakes have this ability or are related to dragons? Reading about dragons in Korean mythology, I found a few references to Imugis, "lesser dragons", which are said to resemble giant snakes and to be able to transform into dragons under certain conditions. But they are also said to be benevolent or to bring good luck, so I find it unlikely that the evil snakes in this story are Imugis. The only linked page I saw for further info on Imugis was in Korean, so I'm not sure if it's possible that these snakes are Imugis or not.