[Ellie May]

It should be changed over a 3-4 day period "minimum" and preferrably 7 or more days.

First it is 75% old, 25% new, then
50%, 50%, then
25 old, 75% new, then
all new

You may have to crush it up so it is harder for her to pick out the small pieces.
If she is having gastrointestinal upset from it, I'd put her back on the old, wait till she feels better, then switch her slowly. If she gets sick again, she will need a different food...