Well I've been successful with train and plane upgrades this way so now we're trying cruises !
I'll let you know the outcome. In the meantime this is the small print on the acknowledgement email. There's also an Amend/Cancel button in case you change your mind I guess.
"What happens if I'm successful?
If your offer is successful, you will receive an email to inform you as well as a new booking confirmation to reflect the upgrade.
To secure your upgrade, you'll have 72 hours to make your upgrade offer payment.
What happens if I'm not upgraded?
You'll keep your existing stateroom.
Please note, while you can make offers for multiple stateroom grades, each offer will be considered independently and only one of your offers will be accepted. If you do not make your payment within 72 hours, you will no longer be eligible for the upgrade offered, and it may not be possible to reassign you your original stateroom."