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 does the imputed righteousness of Christ enable God to do, and still be just?
What restriction did God make respecting marriages in Israel?
What message announces the judgment hour come?
How was the overthrow of ancient Babylon foretold?
Was the Sabbath intended as a day for public worship?
What will Christ do with the saints when He comes?
What are the meek to inherit?

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