I am currently feeding Flint Diamond brand maintenance food I and I am gonna switch to large breed formula, should I make the switch gradually ?

When we got Rocky, the lady was feeding him Ol'Roy. That was changed very quickly :)

Our vet suggested doing the switch over 1-2 weeks as to not upset his tummy.

Not sure about when doing it with the same brand, but the experts will let you know. :)

In my opinion, it really depends on the dog. I can switch Sandy over to a new food, with zero transitioning and she handles it fine. She RARELY gets loose stools, digestive upsets, etc. from not doing any transitioning. Now if I were to try and switch Blaze over to new food with out switching slow, she'd have some nasty loose stools for a weeks or so.

Even though I know I really don't need to gradually switch Sandy over, I still do if I need to change her food (if there's enough of her old food left).

we switch frequently from diamond maintenance to diamond lamb and rice with no problems from any of our three. The lamb and rice is too rich for ours to feed them it all the time. Good luck!

Our vet told us 50/50 mix for 5 days and then switch to new food.
I think it depends on your dog's stomach. Ollie tends to get an upset stomach if anything changes in his diet. 5 days seems to
work.