L.,
Due to the differences in composition between real breastmilk and artificial formula, these changes in bowel habits are perfectly normal. Breastmilk is naturally easy to digest, and while many b'fed babies (like my first) have multiple bowel movements a day while exclusively breastfed, others (like my second) would only have a bm every few days. Because formula is harder to digest, there is usually more "bulk" to it, which equals more frequent poops. If a formula-fed baby is not having a bowel movement every day, there *may* be cause for concern, because formula can be constipating, while b'milk is naturally laxative. If you still b'feed some of the time (like at nights when you're not working), this may help keep your baby from being constipated, even if she doesn't go all the time. My younger son didn't have a poop every day until I started introducing solids and other liquids into his diet (I exclusively b'fed him, tho--never any formula).
Once-a-day poops are normal (but more or less frequent is not necessarily a cause for concern). What is a cause for concern is if she seems to be straining when she goes, or if she cries, or the poop is hard, or if it has any blood in it. (The blood may be due to hard poop slightly tearing her rectum, or may be due to the formula being too hard on her tummy--consult a doc if you see any blood, because it's not a good thing, but it does happen sometimes, and is usually not serious.) If she strains or anything (more than just slightly red in the face, or grunting), then she's likely constipated. You can try to switch brands of formula, or offer her water or juice at times. Some people might recommend diluting the formula, but do that with care, because you want to make sure she's getting enough nutrients and not just a certain number of ounces.
However, it sounds like everything is normal, because she's going once a day.
Trust your "mommy instinct"!
K.