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 is said of Christ's childhood and early life?
What were these gifts that Christ gave to men?
In what prophecy did Christ recognize Daniel as a prophet?
What reason did Solomon give for urging all to fear God and keep His commandments?
When was there a remarkable display of falling stars?
How have the rich defrauded the laborers?
10. As we near the closing scenes of human probation, why may we expect an increase in demoniacal manifestations?

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