how to migrate the Flexmonkey's ActionScripts to run in FlexUnit?

You don't need to migrate anything.

The earlier FlexMonkey code as well as the current will run in FlexUnit 4 without modification. The only issue that you may encounter is that there is a known bug (waiting for a fix from the authors) where FlexUnit .9 tests (older FlexMonkey tests) run in the newer environment don't complete 100% of the time. Hoping we will see a fix on this shortly.

Mike