It is not necessary to switch to the older child formula unless there are eating issues. Your pediatrician would need to tell you to use it for specific purposes. Switching to cows milk is fine.
To do that you'll need to slowly start when they are about 111/2 months old. Take the formula and add 2 ounces of whole vitamin D milk, NOT lower fat, then the regular 6 oz. of formula, then a week later, if they are doing very well go to half milk and half formula. This stage was the longest for me, I did not move on until they were drinking this really good and not having pooh issues. Then go to 6 oz. milk and 2 oz. formula for at least a week then it is okay to go all milk.
Formula supplies all the nutrition a baby needs so as they start getting less and more regular milk their appetite should change where they want more solid food.
I had a mom at my child care center who didn't realize WIC stopped formula at 11 months, she thought she would get another month of formula and didn't transition her baby to milk right, she could not afford the formula. The baby went cold turkey off formula onto milk. The baby had solid white pooh that was the most horrid, stinky, terrible mess that I have ever seen. It was so h*** o* that child's tummy too. I would actually puke if I was in the same room while the diaper was being changed, much more if I was the one doing the changing. It lasted nearly a month.
Transitioning takes time and formula and milk mixed like this to make it easier on the child.