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.

Where are the Ten Commandments recorded?
By what are all men proved guilty?
5. Does the second, third, fifth, sixth, seventh, eighth, ninth, or tenth commandment indicate the author of the Decalogue?
What will He say to those on His right hand?
What encouragement is given to study this book?
By what means were the three Hebrews protected while in the fiery furnace?
What did Christ say concerning secret prayer?

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