Same day change with overnight layover
 

I'm wondering if anyone has had any experience with the following: I'm planning on booking a multi city itinerary BOS-FLL/overnight/FLL-MSY. This is on the last flight out of BOS, and there's a very good chance I'll make it. However, in the case that I don't, would I be able to SDC onto the direct BOS-MSY the next morning (since technically part of the itinerary is that day)? Or would it be treated as a missed flight and have to standby/refare? I'm Mosaic, FWIW.

Also, I'm curious how this would work in practice - would it be considered a complete trip (BOS-MSY) or would I have to SDC each leg separately?

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Do it with Airport not the Phone Line
 

If you show up at the airport with in 2 hours of your original departure time, the unofficial flat tire rule could be invoked. If you get a good agent, they will just rebook you under INVOL REROUTE for the next day.

If you get a so so agent, they might put you on standby.

If you get a bad agent, they might call the help desk and then try to refare your ticket.

Also don't try this by calling the Mosaic line. Even in an emergency they will try to refare your ticket rather than pushing it through the system as an INVOL REROUTE. I once had an accident while traveling and I had to go by ambulance to the ER and I missed my flight. I called Mosaic, and while they were willing to waive the change fee, they wanted to refare the ticket which is obviously going to cost a boatload to book a new ticket as a walk up.

I went to the airport still with my emergency room bracelet on, and the airport still called the help desk and they wanted to refare the ticket.

I asked, demanded, to talk to a supervisor, and the airport supervisor as expected did fix my ticket.

Usually you are SOL if they call the help desk but still even after sometimes a good airport agent or supervisor can fix the mess (overcharging) the corporate HELL desk may (will!) try to do.

If they try to call the help desk you may even want to try to get another agent, or say you want a local supervisor to handle it.