a way to change the challenge that i started with
since this doesn't seem to be possible, i presume that i need to abandon the current account, and create a new one?
You can change your challenge area at any time by clicking on the text below the banner saying “I’m getting SuperBetter at…” on your HQ (you must be logged in.) This will show your current challenge. Click the grey “edit” link on the right side, and you can select a new challenge area. :)