Why has the church's interpretation of head coverings changed today?
The church's interpretation changed because the custom of that day, in which an uncovered woman publicly signaled prostitution, no longer applies. Since society is not bothered with that cultural practice today, custom has completely changed how these instructions are understood, though the passage remains part of the Word of God.
Les explains that in the culture of that day, a woman going out in public with nothing over her head, when people were used to the veil, would be associated with prostitutes. Prostitutes would flaunt themselves by removing their coverings to unveil their profession. Because that mindset dominated 99.9% of the population, Paul dealt with head coverings under those circumstances. Today, however, we are not bothered with that cultural signal, so custom has completely changed our interpretation of some of these things. Even so, Les emphasizes that the passage is still part and parcel of the Word of God. He also notes the common-sense point in verse 6: if a woman is not covered, let her be shorn or shaved, but if it is a shame for a woman to be shaved, then let her be covered. He connects the man not covering his head to Adam being the image and glory of God, and the woman being created from the man.