It won't hurt to just ask, since they'll just quote you what the difference is--just be sure to specifically tell them that no, you don't want to change anything if they come back and tell you that the price is going to go way up.
It shouldn't matter that it was booked at Orbitz.
Might not hurt to make a second reservation with the correct details so that if something really wacko happens to the first reservation (like they tell you the rate will recalculate to 500% of your booked rate if you return it early and to a different location), you can fall back on the second one, which, while it's bad, might be better than that.
Also, keep checking the rates as the dates get closer. Sometimes they drop if competition gets a little fiercer.