Who changed the Sabbath to Sunday according to the Seventh Day Adventists? The Great Controversy describes Constantine as making Sunday a public festival.


  In the early part of the fourth century the emperor Constantine issued a decree making Sunday a public festival throughout the Roman Empire.


In her early writings we twice see that it was the Pope that changed the day.


  But the Pope had changed it from the seventh to the first day of the week; for he was to change times and laws.
  The pope has changed the day of rest from the seventh to the first day.  


Since Constantine was never pope these 3 passages can not be speaking of the same person.

Who do the Seventh Day Adventists say changed the Sabbath day?
Seventh day adventist would argue that the sabbath has not been changed to Sunday. 

However what you are referring to is when in history did the change creep into doctrine. The information you have is correct, first Constantine made the decree for the festivals on Sunday. This made it easier for christians to simply follow along without fear of persecution.

The pope did eventually make Sunday a day of rest at some point in the future. (I don't have the date) but many christians already were gathering on that day.