Shelley - relax! You'll enjoy your cruise so much more if you don't allow the worries to get to you. I can't think of an instance where your nightmare happened - if you get upgraded, you do not have to pay extra!
I have enjoyed all my cruises, whether service was excellent, good, average or even (in one instance) surly. If one crewmember isn't up to par, others will be. Enjoy the warm air, sea breezes, activities, shows, food, etc. Have a great time!