According to Fromm's website, the seven formulas in the Four Star (including the Grain-Free) are interchangeable due to the fact that they have similar compositions. Some people change flavors at the end of each bag and some switch flavors day to day without any problems!
I'd love to switch flavors more frequently just to change it up for Toki but I feel like we don't get through a bag quickly enough for them not to go bad! I'm kind of curious about switching to a grain-free formula without transitioning even though it is from the same line. I will probably just play it safe and transition gradually over a week.
Difference Between Fromm Product Lines - Fromm Family Foods