There isn't an automated way (that I know of), but it's not hard. Most of the syntax is pretty much the same, and part of the point of OpenFL is that the library calls are all the same.

The one time I tried porting a complex AS3 class over to Haxe/OpenFL, I started by copy+pasting the AS3 source. Then, I spent most of my time changing things like Flash Vectors/Arrays to typed arrays, minor changes to for loop syntax, different type names (e.g. Boolean->Bool), and other differences.

Actually, I think there's a really good summary out there...here we go:
http://haxe.org/doc/start/flas...

You can see quite a lot is the same. And I heard there are some even trying to create auto-converters. But it wasn't too hard to convert in my experience. More tedious than difficult.

Ah, I can see why that might be confusing. I think many of those cases are folks who used FlashDevelop as an IDE for ActionScript (Flash), as was the case for me. However, FlashDevelop is also a top-notch IDE for Haxe, so there's a nice past/present/future thing going on here :)

Nope! Not Flash! Really!

The naming convention can be a bit misleading. OpenFL is not an open source version of Flash. It is a library for Haxe with an API that mirrors Flash's. The output of a compiled Haxe+OpenFL project can be native Windows, Mac, or Linux apps, iOS, Blackberry, or Android apps, HTML5, Flash, and more. The output does not require a VM, a plugin (unless Flash is the target), nor is it wrapped in a VM or projector.

Sorry if this is overkill, but it's a common misconception that I'm hoping to clear up!

I can't speak to the Offspring Fling question (though I share the dev's pain!), but I wanted to clarify the bit about "open source IDEs that exclusively target flash/AIR are a horrible idea." Indeed, that would be a bad idea, were it the case. Fortunately, it also supports Haxe, the open source language which can compile into other languages, like C++ and HTML5, to deploy to a staggering array of platforms.

Just wanted it to be clear that we're not trying to keep Flash on life support, here. This is about the future of non-proprietary, multi-platform development!