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.

What is one purpose for which the Holy Spirit was sent?
After man had sinned, how did God show His love, and His willingness to forgive?
What will God allow to come to those who reject truth?
To whom does vengeance belong?
Through what is the knowledge of sin obtained?
Why was Christ declared worthy to open these seals?
Do we always know what to pray for?

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