Reading this thread brought back so many memories. :)
I used to collect Missingno.s myself. Mine never became any other Pokemon. However, when I traded one to Gold version it became a Stantler (spelling? Right name? Can't remember... that darn deer thing!) that knew fly. o.O I was amazed. My bet is that, when you trade it to a different game- it becomes the Pokemon that the number matches. Of course, since I'm not a glitch master... I could be wrong.
Though, I stopped catching Missingno.s and the Lv 100 Pokemon after I realized it was messing up my games. My red version stopped saving and was stuck on the end of the game like... forever. And, when I decided to try restarting... I couldn't save it so I had to start a new game everytime I played. My blue version said that I beat the entire League with my Raichu or something like that. Which... I didn't. And, after a while, my blue version started freezing every now and then. Some people don't see glitches at all. Some people do.