Adam and Eve started off in the garden. They had a certain destiny that God had open to them. Part of this was to be the maintainers of the Garden. When Adam and Eve fell, their calling changed, because they no longer had access to the Garden. Now this is in a negative light, but it doesn't always mean our callings will change because of what we do.
What about Adam and Eve's sons Cain and Abel? God's original calling for them may have been to work part of the Garden as well; if things had gone right. But because a situation changed with people outside of themselves, the calling in their lives changed.
Now put this into perspective with our own lives. God may start us off in one location, but because of the choices other people make, God may request us to move to help in a different situation. We may have originally been asked to live in Canada, but since something starts happening in England, and God knows that we would be suited well to deal with the issues there, he requests us to go there and help. A person may have been originally called to be a pastor, but maybe he would get called to be an evangelist or missionary because a strong need arose due to people's choices.
Our actions also affect the callings of others, and others' choices affect our callings. God may not call someone who came from a broken home to the same level of leadership and role that he would have been if he had been raised in a christian atmosphere. Why? Because the former has a lot of baggage that he may spend years of his life having to deal with; the one in the christian atmosphere may get saved 10 or more years earlier and have a strong foundation. We could say something really hurtful and mean to someone that pushes them over the edge and has them leave the church for a year, or we could say something nice and it be the one thing that pushes them over the edge to stay in the church and keep going. Some of their destiny in God could be destroyed or fulfilled because of our actions. And so it is with everyone. Our callings aren't static, we live in a fluid world where God has to deal with people's choices and move people around according to what is needed. This is not to say that people's inherent gifts will just disappear and be replaced by something else from God, as each person has core features that make them who they are, but it does mean that the rate at which they use those gifts, and where, could be affected by the world around them.
