I am assuming you were satisfied with Category 11 when you originally booked. So if you can't upgrade at the port, don't be disappointed; you still have what you originally wanted. You also probably looked at the other categories, so you have an idea of what they cost (if not, compare categories on the same cruise in 2006).
Get to the port early and check in at the regular desk. Then go to the desk at the far (your) right. Let them know your current state room, and ask if any upgrades are available. They will let you know if available and the cost. Set some arbitrary limit on how much you would be willing to spend to upgrade. If they have a room in a category within your price, then go for it.
We did this on our last cruise. We were in category 4, and inquired what the cost would be to upgrade to category 3. The cost was about $800 (7 night cruise). We didn't feel like we needed to spend over $100 a night for the bigger room. Others would probably jump at the opportunity (originally the Category 3 was more than twice that amount more). It all is a personal decision.
A final thought, if only category 4 or 5 is available when you first check in, and the amount is too much, check back later. Someone in a category 7 may have upgraded to the 4 or 5, then the 7 would be available.
Good luck in what ever you choose!