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.

Why did God confide in Abraham, and commit sacred trusts to him?
To whom will the dominion finally be given?
Under what threatened penalty is the worship of the image of the beast enforced?
What will this prophet do when he comes?
How was Christ strengthened after His forty days' fast and temptation in the wilderness?
With what should our prayers be mingled?
By what have these things been revealed to us?

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