If they are placed at the same depth and in the same position relative to the center of the fixture then they should receive the same level of light intensity as before.
I.e., the PAR value will be almost identical at a location 2" forward and 3" left from the center of the fixture at a depth of 8" of water no matter how far the body of water extends beyond that point.
The only possible decrease in light intesity would be from the loss of any additional reflective bounce off of your substrate due to the additional inches of depth.
I would think that will be such a minimal amount that it won't be a factor anyway.
Of course, if you can borrow a submersible PAR meter you would be able to know exactly how much if any difference it makes at all.
Don't forget to acclimate your corals to the new MH
when you get 'em.