Here is a real-world example during World War I. The German ship SMS Geier sails to neutral port in Hawaii in 1914, territory of the United States. Since the USA and Germany are not at war, the ship is interned. Presumably the crew are interned as well. Then, three years later in 1917, the United States declares war on Germany. What is the legal status of those original sailors?
Do they automatically become prisoners-of-war? Is there another legal mechanism in-place for this?
Assume that both warring parties are signatories to the Geneva Conventions, and follow them; as well as any other major international treaties and agreements that relate to diplomacy and war.