This is how it goes with HAL - the rules vary depending on many things.
If booked under the 5 day flash - no change at all.
Whether and how much on other cruises depend on category booked, promotion booked, length of cruise, how close in cruise was booked.
They seem to have a lot of variables therefore, my best advise would be to be very careful.
In saying that, on a group I had earlier this year, there was some roomie changes and I had no problem doing these.