I've just looked at a heap of forums with people saying the same thing and everyone is saying the same thing, that it can be changed after 2 weeks.
I guess you want to be getting cracking now though, one person said this:
I did the same thing. Here is what you do. Redo your test or take a new one. This time hold something that weighs more than what your goal is. It will tell you that you reached your goal and want you to set a new one. So set the new goal to lose the amount of weight your holding. Then take another body test without the weight. Thus reaching your goal again. Finally, re-set your goal to what you really want using your current weight/bmi. Because I did this all the same day my previous tests were over written. My graph is unaffected. However, if you correct this goal on another day, your graph will have a hickup, but who cares.... you'll be able to save your profile, and just move on. If your situation is opposite, and you meant to gain instead of lose. You must either use a chair to create less weight during the body test. Or possibly ask a lighter person to do the test for you!
That's pretty long winded though.
Are you able to delete your profile or Mii and start again?