J.C.
I used Enfamil, but my boys got both the breast and formula from birth so it was never an issue getting them to switch back and forth. I would try what I did when switching from formula to milk. Start by mixing mostly Breast milk with a small amount of formula in a sippy. Slowly over the next couple of weeks increase the amount of formula so she can get used to the taste change slowly over time. Try having someone other than you do the replacement feedings so she does not try to guilt you into caving in and giving her the breast.
Be prepared to face this challenge again in 2 or 3 months when you try to switch from formula to milk.
Best of luck