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?
Into whose name are believers to be baptized?
Who has borne our sins, and stands ready to help us?
With whose blood was the new covenant dedicated?
In religious things, to whom alone are we accountable?
What does He say of the teachings of enchanters and sorcerers?
What will follow this great conflagration?

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