Why should ANYTHING 'change'?
SUREly the church did not NEED to differ from what Christ set up; so why DID it?
Any teaching takes into account the changed culture. Now we understand astronomy and medicine differently than 2,000 years ago, so the Church needs to explain the Catholic view in the light of these discoveries. On the subject of the veneration of saints, this is what changed: some people died for their faith thus proving it, while others denied Christ. So it became clear that not everyone who goes in church with you and professes a belief in words should be properly called a saint. So the word "saint" came to denote a martyr, and after than any other person whose sanctity cannot be doubted due to the virtue of his life. Of course only the totality of one's life provides such basis for admiration, and so necessarily and logically, veneration of saints who fell asleep in the Lord developed.
Mind you, it was not top-down; plain folks wanted to venerate certain people as saints. The Church recognized this desire as a form of worshiping Christ, the Font of all holiness, and shaped it up properly.