They almost certainly can't just unilaterally throw everything that was previously on one service onto another; the only options are probably to either shut down the MGM+ service and not have stuff on it available at all for a while or to renegotiate a lot of contracts before merging them. (They may still make contracts for new things like this to help justify the service's existence in the meantime, but with provisions that would allow the content to be moved once all the renegotiations are done.)