Okay, let me start off by saying that I have a huge problem with cow's milk being given to human babies. So, this may not be the most impartial advice that you'll get. : )
Cow's milk is one of the leading things that humans are allergic to, and many people do not even know that they are allergic. It can cause very vague symptoms like gas, skin problems, occassional diarrhea, etc. Most people chalk it up to "something they ate" or a simple virus, etc.
I was unable to breastfeed either one of my children and was unaware of milk's effects when my son was born. He drank a lactose-free formula because he had a horrible time trying to digest regular formulas. My daughter was on a soy formula the entire first year. It was only later that we put all of the information together, but my son has suffered from colon problems his entire life. We have to work constantly to keep his intestinal tract balanced and working smoothly. My daughter has never had any intestinal problems. Do I blame the milk? Absolutely, but I know that there are bound to be other contributing factors as well.
We are now a milk-free house and everyone is doing much better. My kids use soy milk on cereal and drink soy shakes made with soy milk for snacks. And yes, they like them. : ) Your daughter's digestive system is used to the soy formula and may become very upset if you change now. Not everyone is allergic to milk, but if things are going well with your current nutrition plan, then why risk it?
Donate the regular formula that you got at a great deal to the Rescue Mission family shelter or to the Food Bank. Someone will be very grateful to receive such a generous gift.