Wasabi!
I used to have a 75G in which I upgraded from 175 MH to 400MH 20K XM's (mogul). I upgraded mainly to get more color and more growth. I had my 400s 3" from the water and most of my SPS were about 5" to 2" from the water surface. I slowly acclimated my corals to the 400s over 6 weeks by playing around with the light periods as opposed to the traditional methods of elevating at first, and slowly lowering the pendants. I also had 260watts of PC Actinics as blue supplement back then! I know it was an over kill, but I thought the more the merrier back then!
This is how I did it:
(actinics were always on for 12hrs/ day: an hour before and after MHs)
WK1 MHs: 2hrs ON/ 2hrs OFF/ 2hrs ON/ 2hrs OFF/ 2hrs ON (6hrs total 400s photoperiod)
WK2 MHs: 3hrs ON/ 2hrs OFF/ 3hrs ON/ 2hrs OFF/ 2hrs ON (8hrs total)
WK3 MHs: 4hrs ON/ 2hrs OFF/ 3hrs ON/ 2hrs OFF/ 2hrs ON (9hrs total)
WK4 MHs: 5hrs ON/ 2hrs OFF/ 4hrs ON (9hrs total)
WK5 MHs: 6hrs ON/ 1hr OFF/ 3hrs ON (9hrs)
WK6 MHs: 9hrs ON
1.5hrs of actinics before and after 9hrs of MHs.
I still moved a few LPS and softies down in the tank, but remember that none of the SPS or rics had any issues with the MHs. Some zoos, like the bambams initially closed up under the MHs, and since they were encrusted on the big rocks, there was no way of moving them. I thought that they would melt away, but to my surprise, they ended up loving the 400s and till this day, I haven't had them open up as huge as they did then under the MHs. I also added extra lugol's solution (iodine) to help in the acclimation. Some people will disagree with me, but I doubled the recommended dose.
I hope this helps.