Free upgrades (other than those advertised with sales bythe cruise lines... which are kind of suspect too I think) are very rare these days, other than a change of decks within the same category.
Many lines have no gone to the option of contacting booked passengers to offer a higher category cabin for less than it would have cost at time of booking (upsell). They offer customers " a deal" to upgrade in an effort to increase revenue.
I've had my own theory on free upgrades for awhile now; I believe the cruise lines offer them to first time cruisers more than they do to repeaters on their line. The loyal customers are already loyal. Impress a new customer and you're more likely to make them a returning customer.