How did this change in observance of days come about, suddenly or gradually?

Answer

Gradually.
NOTES - "The Christian church made no formal, but a gradual and almost unconscious
transference of the one day to the other." 'The Voice From Sinai," by Archdeacon W. F. Farrar page 167.
This of itself is evidence that there was no divine command for the change of the Sabbath.

Where again in the Psalms is His resurrection foretold?
Was it God's design that all should possess the same gifts?
What did Daniel say the king had seen in his dream?
In this covenant with Israel, what obligation was imposed upon the people?
On what day did Paul and Barnabas preach at Antioch?
20. When and by what church council was the observance of the seventh day forbidden, and Sunday observance enjoined ?
Who is it that blinds men to the gospel of Christ?

Questions & Answers are from the book Bible Readings for the Home Circle