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 effect does obedience to the truth have?
How many will yield to the demand to worship the beast?
Which commandment alone of the Decalogue reveals the true God and Author of the law?
What scene in heaven was presented to the prophet as the seventh trumpet was about to sound?
What will be the final doom of Satan and his angels?
Whom does the Lord chasten?
How many foundations has this city?

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