THE ROMAN EMPIRE FOLLOWED SUN WORSHIP

Source: Wikipedia Encyclopedia

Although this does not indicate a "change" of the Sabbath, it does favor a different day for rest, in the cities at least, over the Jewish Sabbath day.

The dominant religions in the regions of the world where Christianity was developing were Pagan and in Rome, Mithranism, specifically the cult of Sol Invictus had taken hold. Mithranism met on Sunday.