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.

How thoroughly does the Spirit search out the hidden treasures of truth?
What does the sorrow of the world do?
What enabled Abraham to endure the test?
22. At the death of Christ, what miraculous occurrence signified that the priestly work and services of the earthly sanctuary were finished?
What is said of the stability of God's character?
What is set forth as the basis for hope and comfort?
How were their prayers answered?

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