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 followed immediately after the gaoler and his family had accepted Christ as their Saviour?
What will be the fate of those who do not obey the gospel of Christ?
What did the prophet say His name should be called?
What event does he speak of as immediately following these times of refreshing?
What, therefore, did God say He would make?
What was done with the blood of the goat upon which the Lord's lot fell?
What do our transient afflictions do for us?

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