Short explanation: Regulations change (as they should), but doctrine never does (because it can’t).
More complete explanation:
There is an important distinction between the Church’s doctrines (which are true statements of immutable fact) and regulations (which are changed as needed for the common good).
Catholic doctrine can be clarified but never changed or overturned, since it is simply a statement asserting an objective truth. Catholic doctrine therefore is true for all people, regardless of their religion, just as 1+1=2 for all people. An example of doctrine is the objective, historical truth that Mary, the mother of Jesus, was bodily assumed into heaven at the end of her earthly life. This is called the doctrine of the Assumption, and is a fact of history. Although the doctrine was first officially proclaimed by Pope Pius XII in 1950, he did not thereby create that doctrine or change it; he only stated that it was already a part of the divine revelation called “Tradition”. Doctrines are not rules or opinions; they are statements of fact.
Catholic regulations, on the other hand, apply only to Catholics, just as any family’s special rules apply only to members of that family. Regulations are not statements about immutable truth. An example of a church regulation is the requirement to abstain from eating for one hour before receiving Holy Communion. This regulation has changed several times in my own life: we used to be required to fast from midnight. Then it was changed to three hours before Mass. Now it is one hour before Communion. Regulations like these can be (and often are) changed by the pope, the Vatican, national councils of bishops, local bishops, and even local pastors. Like traffic laws, they are changed according to the needs of the common good. Regulations are not statements of objective fact; they are rules based upon the particular needs of the people of each particular place and time. As needs change, regulations change, as they ought.
Bottom line: The pope can (and ought to) change regulations, but he cannot change doctrine.
POP QUIZ: Are the “Ten Commandments” doctrines or regulations?
Answer to pop quiz: The way the Commandments are worded gives away the answer. Notice that it does not say “It is forbidden to kill” or “You must not kill ” or even (as many people think it means) “You should not kill”. No, it says “You will not kill.” (“Shalt” = “shall ”, not “should”). It is not a regulation; it is a statement of FACT: YOU WILL NOT KILL. Therefore, the Ten Commandments (despite the name) are doctrines, not regulations; they are truthful descriptions of how God’s people act. “So, what if I do kill?” Then you have stopped fitting God’s description of His people, and have thereby defined yourself out of His people, just as a citizen who starts murdering people removes himself from the group called “citizens” and thereby loses the benefits of citizenship.