Your catechism is not infallible and is subject to change.. as your church wills
However a council is considered infallible...so this can not be changed
RnMom, this is incorrect.
No everything said in a Council is considered infallible. LINK. You have to distinguish between a theological opinion (theologuemenon), Ordinary Magisterium, and Universal Magisterium.
I realize this seems technical, and it is. But the simplest way to put the point, is that only some part of a document which is specifically a solemn definitions concerning faith or morals approved by the pope, and which are not merely temporary or local but meant to be binding on the whole Church, are infallible.
It's flagged with introductory words of the utmost solemnity, like,
Accordingly, by the inspiration of the Holy Spirit... for the honor of the Holy and undivided Trinity,...by the authority of Jesus Christ our Lord,...We declare, pronounce, and define that this doctrine is revealed by God and therefore to be believed firmly and constantly by all the faithful... Hence, if anyone shall dare to think otherwise, let him know that he has separated from the unity of the Church...
Or words similarly expressing infallible intent.