Have your TA (or PVP) mark your booking as "No Upgrade".
This should guarantee that you have the cabin you want.
I'd still check online every once in a while to make sure the cabin is still yours! We were "upgraded" (not on Carnival) once when we had booked a specific cabin. We had to call our TA to get our cabin back!
Have a great cruise!
"A truly happy person is one who can enjoy the scenery on a detour."