We tried switching Lydia over at 11 and a half months, but she didn't like it, so we did it gradually...
1st week we gave her bottles made up with 7oz formula + 1oz cows milk mixed together - just shook it up and it mixed fine.
2nd week we gave her 6oz formula + 2oz cows milk
3rd week we gave her 5oz formula + 3oz cows milk
4th week we gave her 4oz formula + 4oz cows milk
5th week we gave her 3oz formula + 5oz cows milk
6th week we gave her 2oz formula + 6oz cows milk
7th week we gave her 1oz formula + 7oz cows milk
8th week onwards - totally cows milk.
So that process took us up to 13 and a half months.