Our physiologically doesn't not manage change all that well. It increases bodily and mental stress, and doesn't matter whether it's a "good" or a "bad" change. Physiological and psychogical measures often show the same amount of stress whether the change is the death of a spouse or getting married.
Often, we don't know what factors will come with a change, so we have a hard time preparing. Many people also tend not to trust themselves and want to set up fixes before they can identify the new problems, which is impossible.
Also, very few people have practiced skills in relaxation and problem-solving, so increase their vulnerability to change in general, and the unexpected in particular.
If you're like most kids, too, none of the decisions for these changes were up to you.
Finally, humans often focus on their losses when a change comes, and that fixation pulls our energies and attention away from learning what is new and how to effectively respond.
Keep whatever you can the same: Stay in contact with good friends, find some foods that will be the same; find other Americans to socialize with when you arrive, etc.