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.

Upon what basis has Christ taught us to ask forgiveness?
What was to he the character of the little horn?
Why do the inhabitants of heaven worship God?
What does the Sabbath commandment require?
Against what three things does the third angel of Revelation 14 warn men?
Whom does the Lord chasten?
What will finally he the privilege of God's children?

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