Hi avsfan987,
To start, GMAC's policy on rescheduling an Official Test requires that you make that choice at least 7 FULL days before your Exam Date. At this point in the calendar, are you actually 7 full days ahead of that appointment? If you are not, then you cannot reschedule this current date; there's no harm in taking the GMAT as scheduled though (since you can always cancel the result if you are not happy with it).
In answer to your immediate question: assuming that this recent 550 practice CAT Score is a realistic assessment of your current ability level, then you are closer to a 600+ than you probably realize. You might actually have the ability to earn that type of Score right now, but you're not approaching the overall Exam in a consistent, organized fashion to pick up those additional points. On this recent CAT, how many questions did you get wrong because you made a silly/little mistake....?
Assuming that you might want to continue studying for another 3-4 weeks, before I can offer you the specific advice that you’re looking for, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) Over the last 4 months, how many hours have you been studying each week?
2) How have you scored on EACH of your CATs/mocks (including the Quant and Verbal Scaled Scores for EACH)?
Goals:
3) Is your goal score 600 or something higher?
4) When are you planning to apply to Business School?
5) What Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich