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.

What unites all believers under the new covenant?
8. Did God bless and sanctify the seventh day while He was resting upon it, or when His rest on that day was past?
What did some of the people do on the seventh day?
22. What did Augustine, the father of this theocratical or church-and-state theory, teach concerning it?
Upon what one fact does Paul base the Christian hope?
22. When surrounded by the Syrian host, what did Elisha say, and for what did he pray, to encourage his frightened servant?
Why are its gates not to be closed?

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