The following two quotations appear in the same document (The Interpretation of Dogma, penned by the wonderfully titled The International Theological Commission), so they can obviously peacefully co-exist. I am unsure exactly how that is the case, however, so if someone could tell me that would be great!
...the Church condemns anyone who sets that meaning aside under the pretext and in the name of superior knowledge or because of advances in science, or because of some alleged more profound interpretation of the existing formula, or a refinement in the scientific approach to the matter (DS 3020, 3043). Such an irreversible stance and the denial of the possibility of fundamental change is implied in the doctrine of the infallibility of the Church as guided by the Holy Spirit, with particular reference to the role of the Pope in matters of faith and morals (DS 3074). This is based on the fact that the Church, through the Holy Spirit, shares in Gods truthfulness, which cannot deceive us any more than it can be self-deceptive in God himself ("qui nee falli nec fallere potest", DS 3008).
The Church is holy, but at the same time a Church of sinners, and for that reason human traditions can slip in which diminish the one apostolic tradition in the case where the nucleus is violated by a certain exaggeration of certain aspects. And that is why the Church always feels the need for purification, penance and renewal with regard to the traditions in her (LG 8). The criteria for judging such a "discernment of spirits" flow from the very nature of Tradition.