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Again, they cannot do that. An infallible teaching of the Church will never contradict a previous infallible teaching of the Church. That is one of the special protections God has provided for His Catholic Church.

Try this one on for size.

The Catholic teaching that outside the Catholic church, the Roman one at that, there is no salvation.

This is a good place to start.

• Pope Boniface VIII, Unam Sanctam: “We declare, say, define, and pronounce [ex cathedra] that it is absolutely necessary for the salvation of every human creature to be subject to the Roman Pontiff.” "If, therefore, the Greeks or others say that they are not committed to Peter and to his successors, they necessarily say that they are not of the sheep of Christ, since the Lord says that there is only one fold and one shepherd (Jn.10:16). Whoever, therefore, resists this authority, resists the command of God Himself. " — Pope Boniface VIII, Unam Sanctam (Promulgated November 18, 1302)
http://www.fordham.edu/halsall/source/b8-unam.html

• Fifth Lateran Council: Moreover, since subjection to the Roman pontiff is necessary for salvation for all Christ's faithful, as we are taught by the testimony of both sacred scripture and the holy fathers, and as is declared by the constitution of pope Boniface VIII of happy memory, also our predecessor, which begins Unam sanctam, we therefore...renew and give our approval to that constitution... Fifth Lateran CouncilSession 11, 19 December 1516,
http://www.piar.hu/councils/ecum18.htm

• Pope Eugene IV and the Council of Florence: "The sacrosanct Roman Church...firmly believes, professes, and proclaims that..not only pagans, but also Jews and heretics and schismatics cannot become participants in eternal life but will depart `into everlasting fire...unless before the end of life the same have been added to the flock; and that..no one, whatever almsgiving he has practiced, even if he has shed blood for the name of Christ, can be saved, unless he has remained in the bosom and unity of the Catholic Church.”— Pope Eugene IV and the Council of Florence (Seventeenth Ecumenical Council), Cantate Domino, Bull promulgated on February 4, 1441 (Florentine style), [considered infallible by some]

• The COUNCIL OF CONSTANCE under Pope John XXIII condemned the proposition of Wycliff that “It is not necessary for salvation to believe that the Roman church is supreme among the other churches.” [inasmuch as it would deny the primacy of the supreme pontiff over the other individual churches.] — Session 8—4 May 1415;
http://www.ewtn.com/library/COUNCILS/CONSTANC.HTM

• Pius 9, Quanto Conficiamur Moerore: “Also well known is the Catholic teaching that no one can be saved outside the Catholic Church. Eternal salvation cannot be obtained by those who oppose the authority and statements of the same Church and are stubbornly separated from the unity of the Church and also from the successor of Peter, the Roman Pontiff..”
-http://www.papalencyclicals.net/Pius09/p9quanto.htm

• St. Thomas Aquinas: It is also shown that to be subject to the Roman Pontiff is necessary for salvation. For Cyril says in his Thesaurus: “Therefore, brethren, if we imitate Christ so as to hear his voice remaining in the Church of Peter and so as not be puffed up by the wind of pride, lest perhaps because of our quarrelling the wily serpent drive us from paradise as once he did Eve.” And Maximus in the letter addressed to the Orientals [Greeks] says: “The Church united and established upon the rock of Peter’s confession we call according to the decree of the Savior the universal Church, wherein we must remain for the salvation of our souls and wherein loyal to his faith and confession we must obey him.” — St. Thomas Aquinas, Against the Errors of the Greeks, Pt. 2, ch. 36
http://dhspriory.org/thomas/ContraErrGraecorum.htm#b38

• St. Ambrose, "Expl. of Luke: "The Lord severed the Jewish people from His kingdom, and heretics and schismatics are also severed from the kingdom of God and from the Church. Our Lord makes it perfectly clear that every assembly of heretics and schismatics belongs not to God, but to the unclean spirit." — St. Ambrose, "Expl. of Luke", ch.7, 91-95; PL 15; SS, vol. II, p. 85, (quoted in The Apostolic Digest, by Michael Malone, Book 4: "The Book of Christians", Chapter 2: "Those Who Reject Christ's Church are Anti-Christian").http://www.geocities.com/Athens/Troy/6480/catholics/apostolic4chp2.html

Fourth Lateran Council (1215): "There is but one universal Church of the faithful, outside which no one at all is saved."

Pope Boniface VIII, Bull Unam sanctam (1302): "We are compelled in virtue of our faith to believe and maintain that there is only one holy Catholic Church, and that one is apostolic. This we firmly believe and profess without qualification. Outside this Church there is no salvation and no remission of sins, the Spouse in the Canticle proclaiming: 'One is my dove, my perfect one. One is she of her mother, the chosen of her that bore her' (Canticle of Canticles 6:8); which represents the one mystical body whose head is Christ, of Christ indeed, as God. And in this, 'one Lord, one faith, one baptism' (Ephesians 4:5). Certainly Noah had one ark at the time of the flood, prefiguring one Church which perfect to one cubit having one ruler and guide, namely Noah, outside of which we read all living things were destroyed… We declare, say, define, and pronounce that it is absolutely necessary for the salvation of every human creature to be subject to the Roman Pontiff."