I don't understand at all what you are saying. If you agree that the 'truth' doesn't change, what is the problem with adhering to the time-honoured teaching of the Catholic Church regarding what is 'true'? Homosexual activity, for example, has always been condemned as greviously sinful by the Catholic Church and even prior to its inception according to the Law of Moses.
If this condemnation explicit in the Word of God (both Teastaments) is the 'truth' then it can never be revoked. How do we grow to a greater understanding of it? What do you mean by, "... it is we who change"?
Do you mean that because the 'world's" understanding changes, the Church's ought to keep in step with it which I suspect you do? If so, how do you reconcile this with your statement that the 'truth' doesn't change?
I simply don't understand what is meant by, "The 'truth' doesn't change when we grow to greater understanding. It is we who change."