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.

After receiving a knowledge of the truth, what must one do in order to be sanctified by it?
9. What testimony did the Apostle Peter bear concerning his experience on the mount of transfiguration?
By what are all men proved guilty?
How did death enter the world?
What is Noah called?
What does He indicate as one of its purposes?
When Christ sent out the seventy, what did He tell them to do in the cities whither they went?

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