Syntax
So the first thing you may have noticed is that everything in AS2 that had an underscore in front of it no longer does in AS3. That is a MAJOR issues for all of us lazy coders that just want to copy and paste old AS 2 code into an AS3 document – it will fire all sorts of errors such as this one:
Warning: 1058: Migration issue: The property _x is no longer supported. Use the DisplayObject.x property instead.