I have always been told to be extremely careful when changing a horse's feeds because they have very sensitive stomachs and digestive tracts. Changing their feeds too often or not doing it gradually enough can give them colic
or at least this is what we were told when we bought our two horses from their previous owners.
We kept them on the feed that they were on originally when we bought them, but then that feed became very difficult for us to find locally so we had to change it over. When we changed it, we found something nearly identical in terms of the grain, vitamins, and supplements that they were already on.
Even still, we rationed out the remaining feed that we had, mixing at first the majority of the old feed in with a little bit of the new until we had gotten to more of the new feed and a little of the old, and finally just had them on the new feed. We took a couple of weeks to do this though, but like I said, we didn't want to make them sick!
I am not an experienced horse owner though, so I would say wait and see what other members here who are more knowledgable have to say, look up some info for yourself, or even call your vet to make sure.