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 is Christ's second coming described in the Psalms?
How, according to prophecy, was Christ to conduct Himself when on trial?
Whose presence does the Holy Spirit bring to the believers?
What call was made to come out of Babylon?
By what means were the apostles delivered from prison?
Concerning what have we been definitely informed?
What promise is made to them that walk uprightly?

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