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.

Which are some of the most touching and soul-winning of Christ's parables?
In what prophecy did Christ recognize Daniel as a prophet?
How comprehensive are these commandments?
To whom does vengeance belong?
Who alone knows the exact day of Christ's coming?
Until what time were the saints to he persecuted under the Roman power?
While suffering in the garden of Gethsemane, how was Christ strengthened?

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