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 is the whole world called upon to do?
Of what sin were the Amorites specially guilty?
19. What testimony does Eusebius (AD. 270-338), a noted bishop of the church, a flatterer of Constantine, and the reputed father of ecclesiastical history, bear upon this subject?
How does Paul again express this same truth?
How is the church at the first advent described?
What divine law of retroaction attends giving?
Why are its gates not to be closed?

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