I've cruised on the Magic 10 times, and have been upgraded on 3. In each case, the upgrade was from a cat 11 (this is the most common upgrade as they want to have "entry level" pricing available.)
Book a category where you will be happy....any upgrade is then a bonus. If you book low and hope, an upgrade may well not be available to purchase later if the freebie doesn't come thru.
We have booked a 6 down to an 11......for 2 of us, an 11 is adequate, although DD really likes the window. Only you can decide where you need to be for a magical cruise! Book the lowest level that meets your needs (wants), then hope....but don't book a 9 if you really want a verandah. You don't want to be sad about your cabin.