Hang on... Why convert to flv? That's so 2005.

You can do pretty much everything with avi that you can do with flv so why do you need to downgrade? Everyone is slowly dropping support for Flash. OK we all know Apple and the late Steve Jobs didn't support Flash but now even Adobe have dropped support for Flash on Mobile and Tablets. Soon it will be a thing you hear about in museums. Why would you convert anything to flv?

Can you tell us so we can tell you how to achieve the same thing with modern supported technology?