Not a real problem. The disco arm has slightly less drop even with the bottom mount TRE (ball joint in Land Rover speak). If you do make the change, I am interested in your take off arm and the cross tube. I would even trade you for the disco stuff. I should add that you would want the disco tie rod as well (it is where the damper mounts on a disco).
Most people go the other way because the damper on a disco is vunerable off-road to damage.
*not legal advice*