There are a few things you could try. But first I would recommend talking with your vet and asking which one he thinks would be best. Also having a good exam done might help there might be more than one thing going on. The great thing is a phone call doesnt' cost anything.

The two things that you could try are:

1. A probiotic paste or powder, or yogurt This will add good bacteria back to the gut, diahhria can kill off all the good bacteria. Get correct dosage amount from your vet.

2. You can use peto bismol (human or horse type) Just ask your vet for the correct dosage for your horses weight. And with the pepto as soon as the stool begins to firm a little you want to stop the pepto or you can cause them to become impacted.

But first and foremost call and talk with your vet it could be more than just the feed change.

Good luck and Hope this helps