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.

Of whom did Christ say the Scriptures testify?
What treasures are hid in Christ?
How definite should we be in confessing our sins?
What reason did they give for not receiving Him?
From what is He able to keep us?
How comprehensive are these commandments?
What did David ask God to teach him?

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