Hi,
What I did with my DS, who turned 1 at the weekend, was gradually introduce cows milk into his bottles. He had already been having cows milk sometimes in his cereal, and in cooking so I was pretty sure he would be okay with it anyway. I started a few weeks before his birthday, only because I didn't want to buy another tin of formula, and so that by the time he turned one, he would be off the formula completely. Anyway, what I did with his bottles was, instead of making up the usual 8oz of formula, I would make up 6oz and then top the bottle up with cows milk. After a week of this I would do half and half, so make up 4oz bottles and top up to 8oz, then after another week I would make up 2oz bottles and add 6oz of cows milk.
I did this with my first ds with no problem whatsover. With my second ds he started to be sick around the time I started so I was worried that mixing the two types of milk together might have been the problem, but it turns out he was cutting some back teeth at the time, and teething usually makes him a bit sick. After a few days he was okay and is now on only cows milk.
I did post about it at the time, and someone did say that their doctor didn't recommend mixing the two, however, on speaking to my health visitor, she said it wouldn't cause any problems. If you wanted to go down this route, but weren't sure then probably best if you speak to your own dr or health visitor.
Anyway, good luck!
Jo