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 did Christ say to the Jews concerning the study of the Scriptures?
Into what experience are those baptized who are baptized into Christ?
What must be crucified and eliminated from our lives if we would be holy?
What statement shows that sin was in the world before the law was given on Mount Sinai?
What will God's faithful servants be doing at this time?
What answer did Elijah make?
What is an evidence of perfection?

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