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.

In receiving the Son what do we have in Him?
What is required besides a belief in the existence of God?
What danger attends the teaching of false doctrine?
What does this angel say?
What did this nearness of the time of restoration from captivity lead Daniel to do?
What did Jesus say of the sacramental wine?
What courage and strength, therefore, should come to us even in the hour of affliction?

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