I want to switch my horse's feed without her colicing?

Well right now Chance is on Beet Pulp and sweet feed andd has been for a few months. I am moving her to another barn and plan on changing her feed. I want to put her on Dumor pleasure feed and she will be on the gr all day everyday. One thing that worries me to death is her colicing on me. Also i plan on feeding her feed once a day when i move her in a few days. Where she is now she is fed three times a day. Any suggestions on how I can make this transition friendly and easy on her belly? Thanks (: