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 test should therefore be applied to every professed teacher of truth?
What is the true basis of the brotherhood of man?
What prophecy foretold of the disposal of Christ's garments at the crucifixion?
For what purpose is the grace of Christ received?
What is to take place in the days of these kingdoms?
When some went out to gather manna on the seventh day, what did the Lord say to Moses?
What did the Savior say should precede the end?

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