There was a family group on our cruise in 5/01 that had 5 people booked in 2 verandah rooms (don't remember if it was cat 5 or 6) and they were upgraded together into one cat 3 room. They were given the option of upgrading since it did mean combining into one room (albeit a suite) and they chose the upgrade.
We have connecting cat 8 rooms on our 9/06 cruise but I doubt we'll get an upgrade as the connecting rooms are a must and this definitely limits the options for upgrades.