Now that cruises are sailing again, if you want to get an upgraded room for free there is one way to do it.
In the book, The Truth About Cruise Ships by Jay Herring, a former cruise ship officer, a free upgrade can be offered if there's a no show. He says it's the only way he's seen a free upgrade happen. Unfortunately, you won't know if there is a no show until the ship has already sailed.
And the only way to know if there's a no show is by asking. This unfortunately isn't a guaranteed way to snag a free upgrade, but it doesn't hurt to ask!
Source: Express UK