Hi Talky_Tina,
Here is what I know in response to your questions. First, the only difference I've been able to find between a cat 5 & cat 6 is the deck. 5 is one deck higher than 6.
We've never had a problem with noise in our 6 but we've always booked fairly far forward on the starboard side. Let me clarify one thing - you MIGHT notice a tiny bit more noise early in the morning when returning to Port Canaveral because this is the dock side. But you have to get up so early anyway that probably isn't an issue.
I suppose Disney might upgrade if there was a much greater demand for either of those categories than availability but I don't ever remember seeing that particular upgrade mentioned - maybe someone else has.
As for early booking discount the earlier the better. My understanding is that it is generally up until about 6 months (anyone have a better idea please correct me!) but VERY dependent on how the ship is selling. We booked our upcoming cruise 11 months out and received the early booking. DH wanted to wait a bit longer but ship was selling fast. As it was we were wait listed for our preferred stateroom (don't feel too sorry we're right next door to the one we wanted

) and when I called a few days ago to check on the waitlist I was told that cat 6 is totally sold out, 5s are nearly gone, and in fact there were very few staterooms to choose from. In essence this cruise is virtually sold out and it is in June of 2003! Early booking discounts are gone.
I'm sure there are others out there who can add more detail, but hope this helps as a start.
Deb