Forget cross-genre for the moment. What about cross-media works? When is something a poem and when is it a piece of flash? Are there other types of cross-media works other than just prose and poetry?
Movies turning into video games and vice versa.
What exactly is "flash" in this question?
This isn't the same as cross-genre. You're talking about adaptions, which is a different thing entirely. I'm talking about such things as prose poetry, which read almost like flash and, in some cases, could be published as flash.