If she is currently riding in a booster and sits in it properly, then I'd say no reason to change, so long as the belt fit is good and she has a good fit. If what you want is a booster with LATCH, I would try the new Britax Parkway SGL- and they are on sale all over online right now, less expensive even than the Nautilus. I think getting a Nautilus at this point wouldn't be worth your money, because it doesn't fit a lot of kids well as a booster. Fit wise, my 6.5yo JUST moved to the top slots in her Nautilus, she is happily harnessed which is why she is still harnessed in it! She does ride in a Turboboster for playdates wonderfully, so I know she is ready for one full time but she fits harnessed in both seats and WANTS to be, so she is. But when she is finally ready- which I assume to be in the next few months-- I am looking to get her a Parkway for the main vehicle and use the Turbobooster in Dad's car.
The Parkway SGL latches so you don't have to remember buckling in after the child gets out, it has nice features and fits the majority of kids well!
Good luck!