I'd argue for porting to Haxe. Having never used Haxe before, I ported the entirety of an ActionScript MMO client into Haxe in a weekend. It's that easy. Definitely worth the time investment, as it frees you from dependency on a potentially dying platform. The only pain in the language is lousy pascal-style for() loops.
However, be aware that there are limits to what it can accomplish.
For example, the HTML5 compile target requires (because of HTML5 limitations) that you use WebSockets instead of Sockets, which will require some work on the server side, too.
And rich text boxes are a pain when cross-compiling: there's pretty much no web language that has them right, and they all require a lot of work to get anything even close to decent: the Flash Text Engine has not been ported to Haxe to my knowledge, so you can't do anything portable with that.