Melting and Buyback

So this confuses me because I never dared to use it. Can someone explain how this works? Right now I have a ARGO MPUV -> Railen -> Carrack -> BMM. But since BMM is far off, can I de-grade this to get the railen back? And when eventually BMM comes back, get that back too?

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Please help 🙏

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If not ill just have to rebuy the Railen 😄 ..But this would be handy!