The rules specifically allow for re-writing an aspect at a minor milestone, and you get the benefits of a minor whenever you get a higher tier milestone. Also, it is completely legitimate to change an aspect in the middle of a story if the current plot justifies it and the table agrees.
Changing aspects over time can definitely contribute to the growth of a character. For example, I once played a character with the high concept Ignorant Emissary of the Great Phoenix. He was getting strange fire magic from an unknown source, kept getting dragged into adventures, etc. etc. Eventually, he set up a ritual to try to contact the source of his power, and we had a wonderful scene where he interacted with an avatar of the Great Phoenix called up by his ritual. After that, we crossed out the Ignorant part of his high concept.