SDC from 1-stop to nonstop flight on Blue Extra fare
 

Blue Extra fares allow for free same-day changes. They also allow for booking sister cites, so if you're booked from EWR-PBI ona Blue Extra fare you could theoretically SDC (for free) to JFK-MIA flight.

What I'm not sure about is whether you could do the same if the EWR-PBI flight had a stop in BOS and the JFK-MIA flight you're looking to SDC into is a nonstop.

Does anyone know the answer to this?

This is allowed, I've done this kind of SDC many times. The new website doesn't recognize co-terminal cities at all for same day changes, though, so you'll have to contact them.

Super, thanks!