If you have access of raising the lights, farther from source to object, IMHO, I'd go that route. Pendulums work well for this. Doubling the distance from source to focal spot will decrease the amount of par/illumination/radiation by 4. Restricting a corals Zooxanthellae food source isn't beneficial when acclimating a new light source. If raising the equipment is out of the question, then a slow, gradual acclimation is your only choice. If you can go to a craft store and pick up a yard or so of fine vail material, double or triple it up and place it between the lamps and the top of the tank. Run the system as normal, and remove one layer of attenuation per week. This should restrict enough light entering the tank and still provide for the organisms needing it to flourish.
Good Luck