To the question of when life begins: conception.
To the question of changing “doctrines”, we need to research the “doctrine” fully, when you do you’ll find that although it was a “teaching” it was not taught as an infallible teaching. The Church has many, many, teachings which are not infallible, in fact most are not infallible, this does NOT mean we can disregard these teachings, but it does mean they can develop and yes, even change. St Thomas was brilliant, his writings are the basis of many “teachings” of the Church, but he was not nor was his writings infallible. They are wonderful, thought provoking, and insightful, but, only human. Should we disregard the brilliant human fathers of the Church? Certainly not, they inspire us to seek unity with God. At one time the Church “taught” that the world was flat. But guess what, it was not EVER taught as an infallible truth, it was a theory. The Church teaches about the omniscience of God, but, it’s not an infallible teaching, it gives us a wonderful way to understand God, as such it brings us closer to God. One of our problems is trying to determine what is and what is not infallible. If we have a problem with a particular teaching, research it out. Don’t take my or anyone else’s’ word for it’s infallibility, in researching you will find the basis for the teaching, the majority of the time the teaching will have strong Scriptural basis, but not written out in plain language, an example is the perpetual virginity of Mary. The Scriptural references are very compelling, but, no, Scripture does not specifically state she was or was not a perpetual virgin. The Church’s’ teaching of her perpetual virginity can easily be defended on Scriptural grounds. After researching you’ll have a much better understanding of the Church teaching.