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.

After bringing Him to the place of crucifixion, what drink was offered Christ to stupefy Him?
What, in prophecy, is symbolized by waters?
Where did he say he would wait?
By what means were the apostles delivered from prison?
What does Christ promise overcomers?
How does God regard one who abandons his faith?
In whom are we complete?

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