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 relationship to God is established by faith?
In what prophecy was the work of reconciliation foretold?
What was the fourth beast declared to be?
What relation does a justified person sustain to the law?
What does God say the Sabbath will be to those who hallow it, or keep it holy?
How many distinct classes will have a resurrection?
What will cause the Christian to grow?

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