We've never had a set time for bed--it was more like 5 hours after waking from the last nap, or something, so I pulled it back according to how long she'd been awake and if she was starting to act grouchy.
It does take awhile (like, weeks, or even months for some kids) to adjust and its kind of messy in the meantime. Savannah started by skipping her afternoon nap some days, but not always, and then it just got where she was taking it less and less, and her morning nap got later and later.
Of course, night sleeping had always been a nightmare for us, so I have no idea how much was due to the changing and how much was our girl being our girl.
If Gianna is really used to a certain time, you might want to see if you can keep the bedtime closer to that--it may not work, but maybe her night is so set in stone that she'll make up the sleep somewhere else. Whatever you decide to do, just stick with it for awhile before you give up.
Good luck!
~Kim