#1 In 321 AD, Roman Emperor Constantine I issued an edict mandating a day of rest on Sunday, which he termed "the venerable day of the sun." This decree, while reflecting his Christian beliefs and aligning with the Roman worship of the sun god. He did this with close collaboration of Bishop Eusebius. cited in the "Commentary on the Apocalypse" Moses Stuart Vol. 2 9, 40. Andover, Merrill and Wardwell 1845.
#2 See A Doctrinal Catechism by Rev. Stephen Keenan p. 174 claiming Romes authority to change the Sabbath to Sunday
#3 Dr. Butlers Catechism Revised p. 57 More Rome has authority to change Sabbath to Sunday.
Sabbath observance was still going on in AD 364 proved by the fact that the Church Council of Laodicea forbade the observance of the Biblical Sabbath.
Your assertion is a smidgeon of fact with a huge admixture of error and distortion. By this time, the Sunday Lord's Day was well established by Christians, in fact, from New Testament times. Constantine issued his edict (a civil one, not a religious one from the Church or a Pope) as a means to keep unity in his empire by making Sunday the Lord's day a civil law. This was done to combat, among other things, the Sol Invictus cult, not to complement it.
They forbade the Sabbath of the Old Law because it was not binding on Christians. Any Christians that were holding fast to it were doing so in error.