I agree with what Tony said and Steve. This is how I acclimitized my tank from flour....to halides. Worked very well.

For Those Who Have Switched:
How big was the jump in wattages between the FL or PC and the new MH? Does this affect how long the on-off periods are as you ramp up, or is it more the quality/intensity of MH that makes this necessary?

I mean, if a person is going from 220W of PC to 250W of mh, would there be an issue (could you just fire it up)?

yes there would be a issue as the MH is much more intense. all the 250 watt means is how much power the bulb its self consumes to make the light. if you want to compare intensity you can look at Lumin. most PC's are in the 8000 lumin range. whare a 250 MH can be up to 20000 lumin (or higher) depending on the bulb so you can see it is 2 to 3 times more intense than a PC which means more light will penatrate through the water farther.

Steve