Non-Catholics often bring up the issue of disciplines that have changed in order to counter the claim that the Church is guided into all truth, which of course, is unchanging. They say, "If the Church is guided into all unchanging truth, how come before Vatican II it was a sin to eat meat on Fridays? Something can't be a sin one day and then not be a sin the next day. That just doesn't make sense," or, "How can different places require people to attend mass on different Holy Days of Obligation? If it's sinful in one place not to go, it doesn't make sense that it can be ok in another place." What they are missing is the issue of doctrine vs. discipline.
The Church was given the authority to bind and loose, and so they can require certain disciplines if they feel it is beneficial. Let's look at it this way...there are some things that are universally true to all parents and the rules are the same everywhere. For instance, I've never met a parent who allowed their child to run into a busy street (for obvious reasons). That's a universal rule, because it's universally true that running into a busy street is dangerous and could result in harm. Now, there are other rules that are flexible, and it is up to parents to decide...for instance, the appropriate curfew for their children. Once that curfew is set, if a child doesn't come home by then, they're in trouble. Why is that? Is it the actual time itself that has some universal significance? No, because different parents will set different times for the curfew. The reason a child gets in trouble is because of their disobedience.
The same applies to Holy Days of Obligation (HDOs). Now in our big family the Church, there are certain rules that are universal, because they have to do with unchanging truths. Then there are others that are left to decide to the local bishops (they may be things that will be different culturally, etc). When someone willfully ignores a HDO in their area, the sin is that of disobedience. So, it's like if a kid willfully ignores his curfew, he can't come home and say "But Johnny's curfew is later, so it doesn't make sense that I have to follow your curfew!" (Lol, well he might try, but it won't work for any capable parent!) The parent understands it's not the *time* that is the problem, it's the disobedience. We do believe the Church has absolute truth, but what days are HDOs is not a "truth," it's a discipline, which can be changed according to time and place, like a curfew can be changed depending on the age and maturity of the child, etc. There is nothing inherently wrong with coming home at 11:00 instead of 10:00, what is wrong is the disobedience. Likewise, there's nothing inherently wrong with eating meat on Friday, the sin was in the disobedience. Now that it is up to the individual to decide what he will do on Fridays, there is no disobedience in eating meat, and therefore no sin.
Doctrine, on the other hand, is unchanging because it has to do with truth. For instance, the teaching that priests must be men is unchanging, the Church has no authority to change this (as explained by John Paul the Great in ORDINATIO SACERDOTALIS), but whether or not priests can marry is a matter of discipline which the Church does have the authority to change. (Not that I think they should, I've heard too many priests explain how celibacy was such a gift for them.) It's easy to confuse doctrine and discipline, but it's important to differentiate them to truly understand how the Church works.