If the dog is doing very well on the puppy food, you could consider letting her continue on it. People errouneously belive "puppy" food or "acitve adult" food labels really mean much! Or with a dog as small as a Chihuahua you could probably afford a more optimal diet than dog food. You might find reading about natural diets interesting. Articles about it are all over pethobbyist.
Even the very best "premium" dog foods are sub par to the food you could buy in the grocery -fresh meats and fish. With larger dogs the convenience of dog food is great - but with tiny pooches - it's just as easy and not that much more expensive to feed human grade foods. - meats and fats with only very small amounts of fresh veg and fruits - no grains at all.
Perhaps you'd like to consider a raw diet or a "raw meaty bones" type of diet. UNCOOKED bones that are too big to swallow yet small enough to encourage chewing by the dog are probably the thing most likely to extend a toy dogs health. (Tiny dogs are notorious for having periodontal disease). Gnawing on bones can help - big time.
If a tiny dog doesn't like a dog ood there is no reason to continue feeding it.