You're really not supposed to just switch foods. You should give her a mix of new and old & gradually increase the ratio of new food over several days at least.

I won't comment on how fat she is because I don't know why she's fat, but I do want to point out that a lot of those "special formula" foods are complete and utter garbage. Unless there is a medical issue (which, again, I won't comment on because I am not a vet), you are better off going with a higher quality kibble or a well researched raw diet and increasing exercise. When you do start exercising this dog, remember that she will need to be worked up to longer workouts. You can't just take a fat dog on a 5 mile hike right out of the gate just like you can't just take a fat human on a 5 mile hike right away. She must be built up to it. Keeping her fit will also keep her joints in better shape which can ultimately mean you get a few more years with her. Food is not love. Fat dogs are not cute. Fat dogs make me sad. Good luck with slimming her down.

Give a little time. Weight loss food is very different than normal food so the tummy just needs to get used to it. If it doesn't get better, take the dog to the vet and they will guide you to find the right kind of food. Until then hang in there friend and good luck

But does it make the dog lose weight?

That's why you're supposed to start out a new food by mixing it in with the old one so their bodies get used to it

Honestly instead of ducking around with expensive "diet dog food" how about putting her on a healthy home cooked diet, veggies fresh meat and rice go a long way

Of course she's gassy. That's how you know the new food is working.

That means it's disagreeing with her. Get other food that doesn't upset her poor little digestive system.

You gotta give it to her slow by mixing it with the other food at first everyday you give it to her decrease the amount of the other food then stop giving it to her since you switched her quick it upset her stomach just wait a few days and it should start calming down

What's more important to you? Her health or your momentarily happiness??