I too miss the ability to change the dx text while leaving the code untouched such that I could find the dx by MY Description. That "feature", i heard, was blamed on some regulatory issue, but don't take that as gospel either.
You can change the diagnosis test in the Assessment text box, just leave the # and : and parentheses alone. That new descriptive text will stay in your note and appear on your orders, and the code behaves correctly for billing.
Of course if you use ctrl-N to start a new note, your changes will vanish from your current note. (can be copied back from past visits if needed)