I've been blessed (or cursed?) in life with an exceptionally intelligent terrier named Tucker. He has a ton of food allergies, so he has to eat this special dog food with no wheat or chicken products, and years ago we settled on buying him the salmon flavour. Six months or so ago, tragedy struck our household when the store temporarily ran out of his go-to bag. Being naive, I grabbed a bag of white fish food in the same brand instead. Fish is fish, right? NO. No, it isn't.
The first day I served him the new food he flipped his bowl over. The second day when I did it again, he flipped the bowl over and put his food's placemat on top of it. Day three when I was still trying to get away with this whitefish sham he pushed his food bowl across the room to the garbage can and then collected random trash and placed it on top of his food in the bowl. With that, I finally got the message, and I made a special trip to an out-of-town store so his salmon could return.
For an animal that is interested in eating every discarded burrito, chip, and sandwich on the road during our morning walks, my dog has a particularly selective palate, which means getting him to eat anything new is a bit challenging.
He's certainly not unique. Animals, like people, get used to their favourite flavours and can be reluctant to change. When you're actually planning on swapping food (I wasn't) you can try slowly mixing the new with the old so they don't catch what's happening until it's too late.
When you have to make a sudden change, Reddit user ucffool has an interesting idea: crush up treats and put them in an old seasoning bottle, and then use it to "season" the new food with a flavour you know your pet already enjoys.
Their post suggests using and making the seasoning specifically for cats, but I think it would work well for pretty much any animal. Adding a light dusting of that flavour they enjoy could have the same impact as mixing the new food up with the old, and will make transition to something new (or tricking them into eating the only thing they had at the store) and easier proposition.
If the seasoning doesn't work, there's always the first way to every dog's heart: peanut butter.