Sorry, the change you are looking for is from agricultural land to equestrian which does need change of use. It's got nothing to do with arable or grazing - it's what animal you are grazing that is the difference. If they were working horses for farm use, you'd be fine. Looking at some of the "horsiculture" that goes on and the mess the holdings get in, I'm not surprsed the planners get agitated. I'm sure some horseowners do manage their grazing well, but a hell of a lot don't - docks, thistles, broken down fences and field shelters, RAGWORT!
Sorry, rant over.