We do it gradually, too. It's not the brand so much as the switch in ingredients that can cause upset, and new flavors (even within the same brand) usually have new ingredients.
"We are--each of us--dying; it's how we live in the meantime that makes the difference."
"It's not what you gather, but what you scatter that tells what kind of life you have lived!"
"Be kinder than necessary, for everyone you meet is fighting some kind of battle."