My concern would be this:
1) Flash is prevalent today. No matter how much people are bashing it on HN, its widely used especially on the sites which are incredibly popular. 2) HTML5 adoption will be slow. Look at how long it to ie6 to die and you'll have an idea of how long it will take html5 to become prevalent. 3) You're taking a technology that allows you to be accessible today and trading it for a technology which will allow you to be accessible at some unknown time in the future, basically removing any barrier to entry for your competitors.
The hyperbole makes this seem worse then it is, I'm sure. You don't really have motivation to ditch your current flash implementation. You'll just put it into a "support as needed" mode.