DCL's 'official' word on this topic is that they are only valid for the cruise which they were purchased on. They are priced on a per day basis - and must be repurchased on returning trips.
I understand that they DO change the design quite often.
I've heard that they change fairly often, but I've been on two DCL 7-day cruises and the mugs were exactly the same. We didn't bring them back because we forgot about it, but we could have.
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