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.

Whom did God promise to send with Israel to guide them into the promised land?
Is there salvation through any other?
Who is the real power operating through the beast and his image, and demanding worship?
What determines whose servants we are?
How many will see Him when He comes?
How did Christ, from the Old Testament Scriptures, prove the resurrection?
What universal chorus of praise will then be heard?

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