I know that when you change their food you have to slowly mix it so it doesn't mess up their little tummies

and I have recently decided to change to Bil-Jac (which you guys should try it on your picky eaters!!!) Gracie eats this stuff out of her bowl!!! which i can't get her to do with the science diet....but i was wondering how long i needed to mix the two...she really just eats the bil-jac, she eats around the other stuff....so when can i give her plain old bil-jac???
Thanks