The change in kelvin rating and possibly PAR is what caused bleaching. You have to acclimate corals to change like this by greatly reducing photoperiod to only a couple/few hours per day for a couple weeks and slowly increase it.
What wattage bulbs you're going to run depends on your set-up... what's the footprint, how high off the tank are the lights, and what type of reflector/ballast/bulb (SE/DE) are you using?
Coudl you just use 250s? Sure, but you could also just use 175s or 150s or 400s or... it all depends on your set up.