Legitimate in a Flyertalker / travel hacker sense.

Not legitimate in the real world sense that the OP is supposed to be travelling from CPH to BOS. Although BA ought to be happy to get off-the-hook by allowing the OP to simply drop the CPH-LHR leg, the "normal person" solution is to just be rebooked on the direct CPH-BOS with SAS.. (and, if necessary, for the OP to book themselves onto that flight and sue BA for reimbursement).

And of course, this is a good PSA about avoiding codeshares unless absolutely necessary...