I have no idea. The createJS tool is downloadable for free for Flash CS6 and comes with Flash CC. You'll have to try it for yourself. Your file may translate very well or you may have to do a lot of work to get the html version to run.
You can. It's just a bit annoying. See my nearby rant, which has a few things to watch out for, and a few work-arounds...
Lots of mines in a FLA file that was 'already made'. You can make anything work, but it will cost you your sanity.
This describes every HTML5 project: https://www.youtube.com/watch?v=nU2y6ztlMAQ
AS2 will look a bit more 'foreign' than the AS3 code, if you're just porting it straight across. The CreateJS libraries look a little more like the AS3 classes, but are very different to either AS2 or AS3.
AS3 can almost, sort-of work, if you strip all of the type safety off, and translate the classes.
If it was all just 'stop' and 'gotoAndPlay' kinds of junk in the frames, you actually get off lucky; you can write the AS code, then write the JS code in the same boxes. It's still awful, but it's less awful than doing bigger projects.
var title_loop:SoundChannel = (new Title_Music()).play(0,1001);
bNext.addEventListener( MouseEvent.CLICK, continue_prank );
bPlayAgain.addEventListener( MouseEvent.CLICK, continue_prank );
But either way, it's the identical task of taking things that are made to work with Flash, and 'fix them' for the CreateJS libraries.
Oh, and NO, it absolutely won't be 'write once, run everywhere'.
It will be 'write once', then discover how it's broken in endless, subtle ways on each and every browser, and if you're truly unfortunate, someone will be whinging at you about 'old' browsers, like IE9, and even IE8. Yes, it's truly possible you'll hear about something not working in IE8 from whatever you have for 'QA'. Then the bugs about 'iOS' and sounds will start coming in. Oh, you unfortunate people.
People hawking HTML5 tools at you will claim it's 'all platforms', but it's a miserable, endless mess. I'd welcome you to hell, but really, hell would be an upgrade, compared to working with 'web standards'.