Often the changes which Night Flight is referring to just happen. A new pilot who decides the new company SOP's are junk and decides to use the ones from their previous company. Many times this goes unnoticed because of a lack of training (pencil time) or a lack of quality control. This is not typically an issue for international or regional airlines, but more often a problem for smaller mom-and-pop operations. I spend an inordinate amount of my training trying to dissuade new pilots from using their old SOP's.
Journey Man and Zaphod Beblebrox are right: change is good. New SOP's give a fresh perspective. However, undocumented change is not good and I believe that is what Night Flight is probably more looking at.
I did a quick search on the online library of Embry-Riddle and could not find any related, peer-reviewed studies. One or two articles (not studies) on SOP creep, but that's about it.