It is an unpublished benefit that DMs can SDC between co-terminals (i.e. JFK/LGA). MCO and PBI are not co-terminals though. So the only way to do this would be to pay to change your flights or hope for IROPS; during IROPS the website lets you change your destination to a handful of nearby airports, even some that are hundreds of miles away. If the website doesn't let you change to MCO, an SMS agent most likely will.