Thank you very much for the informative reply.
I started the first degree after high school. At that time, in my country, there was no bachelor program, it was a five year program that ended with a masters' degree. However, after that I decided to pursue physics which had been just a hobby before. First, I had to obtain bachelor's degree in physics.
Then I started the master's program in theoretical physics. It had a great freedom to pursue what I liked, so although I was officially affiliated with the theoretical physics department, I did both my theses (bachelor and master's) at astrophysics department, both on the Solar System; at the same time I could attend great courses on computational physics and plasma theory in the theoretical physics department. As I wanted to follow with phd in USA, this course of study seemed great to prepare for that. I saw no point in going to astrophysics department here and then take almost similar classes in the usa.
So for me, the only big change in focus was when I started the bachelor in physics. Since then I just keep focusing on a more and more particular field.
Now, in bachelor and master programs I focused on two similar topics - asteroids. But, in my phd. I want to move further, focus on a bit different part of the solar system evolution. At ucf I am being offered work on asteroids again (because of my background) and because I believe the phd study will shape my future career, I am reluctant as I do not want to focus my career on asteroids.
My dream school is Boulder, Colorado.