One way is to go to the hardware store and buy sheets of window screen. Cut the screen and lay 8 layers of it on top of your tank. Run your MH's for a normal light cycle and every 3 or 4 days remove 1 sheet of screen. After about a month, you'll have full unobstructed light. Another way is to seriously cut back on your light cycle and only run the MH's for a few hours a day. Gradually leave the lights on longer until you're back to your normal cycle.
If possible, you can also hang the lights very high and gradually lower them to where you want them.
It sounds like you're already aware that going from pc's to mh's can fry everything if care is not taken to acclimate to the new light. Also, depending on your livestock, you still might need to move much of it to your sandbed.