Can't help you w/whether you can raise the hood.

But, if you have to change your niche, this may help...I have a Vent-A-Hood and we planned our niche based on what the VAH specs stated, so my niche:
...starts 6" above my expected countertop (approx 36" off the finished floor)
...20" tall (30" wide, 6" deep)
...ends approx 4" below the hood

Here're a couple of pics:

1 1/2" isn't going to make much of a difference. I wouldn't redo anything.

I echo Weissman - 1 1/2" will make little if any difference. Stay as you planned

Thanks everyone. I didn't think 1 1/2 inches would be a big difference. I just realized I'd better try to figure out what code is, though. I don't want to not pass inspection because of 1.5 inches!!!

I had the same problem. I did not account for the arch in my hood so my VAH liner is mounted 33 inches over my range, instead of the 30 inch recommended. It works perfectly, even when the grill is really smoking.

I would leave it as I don't think it will make too much of a difference. Last night, I had 4 burgers on the grill and 2 burners going without any problems with the smoke or grease and I only have the 600 cfm model.

Generally, hoods are not required by code - the only issue would be if you had a wood hood and had it too low - creating a fire hazzard - but that isn't the case in your situation.