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 explains God's apparent delay in dealing with Sin ?
Who appeared to accuse Christ before Herod?
What did Jesus say of the sacramental wine?
7. Because Jesus healed a man on the Sabbath day, and told him to take up his bed and walk, what did the Jews do?
What did the Sadducces in Christ's time deny?
What is said of the strength and character of the angels?
What does the Apostle Peter say concerning the trials through which every believer must pass?

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