What 400,s are you thinking of using?
We had a couple of good threads on here regarding that. There are several ways of ramping up. One thats seems popular is 4hrs. on and 1 off. Seems thats some type of limit before bleaching. I assume its done for several weeks. I would have to go find the threads on RC for more info.
I know the current TOTM there was switched from vho to 400w Radiums. He ran the vho most of the day and the Radiums for a small period of time. I believe he ramped up the 400,s a but more each day for at least a month.
Another way is to have the hood on a cable system, starting high up and coming down lower a bit at a time. That one never worked for me,
but I know others have used it.
Which ever way its done, be very careful and yours corals should be fine. Get in a hurry and its bleach city.