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 record answers to this prophecy?
What did the devout Simeon say when he saw the child Jesus?
What did the prophet see in this vision?
21. What miraculous event occurred at the death of Christ, signifying that the sacrificial system was for ever at an end ?
How does Paul describe this dedication of the covenant?
Did Christ keep the Sabbath?
Before he was afflicted, what did he do?

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