I've never heard of anyone going from a 10 or 11 to a 3 or 4. That doesn't make sense. If they upgrade a 10 or 11 to an 8 (or even a 6), and then upgrade the 8 to a 6 and a 6 to a 3, they have 3 happy families. If they move from an 11 to a 3, only one family is happy.
There are occasionally upgrades available at the port, and they are discounted. However, frequently there are none available. Bottom line is book something you will be happy in....if you get a free or paid upgrade, consider yourself lucky.