You shouldn't change those files to async because then the JS will break. Using async means the files will download and execute in any order. You don't want stuff depending on jQuery executing before jQuery is available.
Potentially you could use "defer" instead, which delays execution until the page is ready and executes them in order. The problem here is that either everything or nothing must be deferred. If only some things are deferred, anything not deferred (e.g. a script from a plugin) could break.
If you still decide to change the JS, currently those files are set up in the core - see the qa_output_content function in qa-page.php. The better (but more difficult) way is to override the head_script function in your theme, detect when a line is a script file, and do some string/regex replacement.
