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 may we know that we are the sons of God?
What blessed assurance is given all believers in Christ?
14. What was the next step in their plan to secure lawful authority to carry out their unlawful purpose?
For how long does marriage bind the contracting parties?
What was Paul's manner respecting the Sabbath?
Why does the Lord call the Sabbath His day?
What will be the fifth plague?

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