I wouldn't switch any food cold turkey, especially in a horse that just coliced. I don't think adding psyllium in should cause a problem but check with your Vet. A phone call to the Vet is free and WAY better than causing a problem that then needs a visit from the Vet.
She may not want to eat the psyllium plain.... my horse HATED it. I mixed it with soaked beet pulp and grain when the Vet wanted me to do a 30day stint of it but my horse was underweight and already getting grain at the time. You can ask your Vet what you can mix it with that would be safe for her (I remember my Vet said no grain for a while the last time my horse coliced).