Why not take it?
I assume they'll match your OW status, so as BAEC Silver = OW Sapphire = Duna Silver / Gold / Tin / Lead / Whatever (I don't know anything about Malev).
Although you may still credit your miles to BAEC, having an "elite" (Sapphire) status with Duna may give you some additional Op-Up potential when flying on Malev Metal.