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 triumphant chorus will mark the end of the reign of sin?
To whom will the gates of the heavenly city finally be opened?
14. What instruction did Abraham give his servant Eliezer when sending him to select a wife for his son Isaac?
What was done with the blood of the goat upon which the Lord's lot fell?
What determines whose servants we are?
To whom, then, should its observance be rendered?
How many does Paul say will suffer persecution?

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