Ultimately, I would leave it alone. If your gravity is way low, adjust with malt extract. If you're gravity is close but you can't fit it in your fermentor, so what?
If you boil again, your bitterness will go up. The problem is bitterness sort of maxes out around 80 IBUs. There are a couple of recent Beersmith podcasts talking about the threshold if you're interested. I don't think you're going to notice a difference in bitterness.
You will probably notice a change in hop flavor. I would take some of the hops planned for dry hopping and throw them in at the end of the reboil. Perhaps, a half ounce of each type with 5 minutes to go and then another half of each at flame out. Then go and order some more hops for dry hopping. You have some time before you need start dry hopping.
(Target OG Points - Actual OG Points)/44*Wort Volume= lbs of DME needed to boost OG
Target OG: 1.088
Actual OG: 1.075
Wort Volume: 6 gallons
(88-75)/44*6 = 1.8 lbs DME needed