Velvet,
The way I did it was similar to what KCW recommends. I removed an ounce, or a half ounce of formula from the df every time I moved the time up, and added those oz to the next day's feed. We weren't on solids back then yet. So by the time we were at 8:30pm she was down to an oz already and one night I just bit the bullet and did not give it to her.
If you're still giving her the same amount at the earlier time as you did at the later time, I would recommend subtracting half oz to 1 oz at a time, adding it to the next morning's feed, and to the other feeds, etc. You should be able to eliminate the df within a week to 10 days.
Hope this helps.