The Pope alone does not make any changes in doctrine. It is the Pope along with the Magesterium, the Cardinals and leaders of the Church, who make changes, or declare a doctrine infallible. There have been very few changes in doctrine, and the basic ones have never changed, though there have been a few added along the way.
There is Dogma, Doctrine, Teaching, and Law.
Dogma can never change, and I hate to tell the Episcopalians, but the Trinity is Dogma, and it can never be changed. Most of the Dogma can be found in the Nicene Creed, although not everything in the creed is Dogmatic.
Doctrines are belief systems that flow from the Dogma and from the scripture. Things that we believe, like the Sacraments. Doctrines are not likely to change. Some of the saints are known as Doctors of the Church because they developed or revealed things that we've found so important as to be doctrinal. Again, Doctrine MAY change, but it is unlikely.
Teachings are the ways in which we present dogma and doctrine to each new generation. These things may indeed change, as it is right to teach the newcomers in ways that they can relate to and understand.
Laws are canonical rules meant to protect the dogma and doctrines. They may be changed, and often should be. Laws are where a church runs into the problem of being pharisaical. The Pharisee (or Rabbinical or today, Orthodox) sect created so many rules as a 'hedge of protection' around the Mosaic Law that they became almost impossible for an individual to keep. Keep one and by keeping it, break another. This is how I fear our country is governed today.
This is just how I learned this in RCIA, and I think it's interesting.