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 God desire to teach the children?
What is the Lord able to do regarding the future?
By what three distinct acts, then, was the Sabbath made?
What prophet had foretold Christ's ride into Jerusalem?
Have the worthies of old gone to their reward?
What was the result of this message?
What was to take place under the third trumpet?

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