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.

When believers are baptized into Christ, whom do they put on?
How did His questions and answers impress those who heard Him?
How are believers scaled?
Was the judgment still future in Paul's day?
How early was this sacrificial system known?
7. What description does Paul give of the sufferings endured by some of God's people in former ages?
What petitions may we confidently expect God to hear?

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