Your experience could be different but when Ive tried this I’ve been offered an upgrade if available but you are paying full guest price for the upgrade and it would’ve been much cheaper to just reserve a veranda at the cast rate when I booked (ie in Alaska I was having second thoughts and asked at port…the cost for an upgrade was almost half the total I paid for our ocean view room, needless to say I ended up keeping our ocean view & absolutely loved our room!).