ActionScript class files can be converted automatically using the “as3hx” tool or converted by hand. I prefer doing it by hand myself (with a few find/replace scripts) since it helps me work through untyped areas of code manually and helps me see problem areas that may require additional attention to port well.
OpenFL supports much of the standard Flash 9/Flash 10 API, though unfortunately this does not generally include the “fl.*” components or “mx.*” Flex Framework classes. There is a bit of work done for fl.* classes here though I am unsure of its stability or completeness.
There is a SWF library that supports SWF assets, omitting most scripts, new tweens, morph shapes and sounds. It has been used most for static assets, and there is a little bit of support for animated MovieClips. I’ve long been excited about the prospects of FLA/SWF support though most of the energy over time has gone toward alternative formats (spritesheets, etc) so animation (especially) has not received as much development work over time.
A feature that I also recommend looking at using (as needed) is the “Export as Bitmap” option within the Adobe Animate IDE. This allows Animate to flatten objects into a bitmap when exporting the SWF, reducing complexity and guaranteeing that rendering will be identical to the original. This is really useful for complex vector shapes that may not perform as well when converted to HTML5 canvas