Slowly switch over, by adding more and more of the new feed slowly, until eventually she's on it fully.
Do keep in mind though-if she's used to sweet feed, she's likely to find the other quite repulsing for awhile until she gets used to the blander taste.
There's a product called Probios that helps with digestion. It would do her good, as extra protection, to supplement her with that while she's making the switch, to make sure everything goes right. Plus, it's good to have around.
I'm not sure why she's on sweet feed if she's not being worked very often. It's a hot feed for hard working performance horses burning a lot of calories at a time... Anyways, do you know what feed you plan on switching her to? Many good ones out there, with good rations depending on the horses size and work level.
"You're just as sane as I am."~Luna Lovegood.