I can only speak from our personal experience, but because our charter was so far out (August 29,) Moorings was under no requirement to do anything for us. If we changed our charter date, it would have been a $1000 change fee. And then we would have been locked into summer 2021, which my parents didn't want to do. And if we canceled, they'd keep the $3500 deposit.

You might be able to get a free change if you were in their no-sail window of 6 weeks before the charter, but then they'd already have your full balance paid so the question of moving the date with only the deposit paid becomes moot.

My parents say there is no way they will travel to Europe this year, so if we waited too long, we worried we wouldn't have the flexibility to cancel with a full credit because right now Croatia is starting to let sailing charterers arrive in the country and sail. It's pretty hard to get there though with only maybe a half dozen European cities getting flights from the US.