If it's possible, go to the passport office in person. If not,
yes, you do have to send your birth certificate, the original, and yes, you will get it back.
If you lose weight, you will not need to get a new picture.
I have changed my hair color and have not had to get a new
picture.
Have a wonderful time in Japan. Japan is one of the places in the world where Americans are welcome. The Japanese people on the whole are very polite and friendly. I'm sure you are going to have an extremely interesting and fun time. See as much as you can. Learn as much as you can. You will remember this time for the rest of your life. :thumbup: