It all depends on your definition of initially. If it means "at first" then that is exactly what has happened. I don't think that BA specified a time scale for MF to be expanded to the 747.
The additional cost imposed upon BA by UNITE's threat of a strike ballot needs to be recovered. This additional cost can be recovered by training MF onto the 747 in order for them to operate some more premium trips. Potentially another own goal by UNITE.
Or it could just be another scare story put out by the union in an attempt to garner more support from within the CC community.