Reset start weight!

Try Settings>Update diet/fitness profile and then changing current weight in there.

I could never reset the start weight.

I cheated - I went back to my first entry and put my new start weight in so that the "JLG1986 has lost __ pounds" line was correct. I do think the option to define a start point would be a GREAT feature for MFP! (for me, I quit for a year and came back heavier. I really wanted to track my weight starting this year...not have it be based off the first weight I put into the system years ago)

You can also go in and delete all of your weigh in history. But you have to keep one day. Alter that day to whatever day suits your fancy and plug in the weight. Done.