No, your FP's are tied to your park tickets, have nothing to do with your reservations, and I would be very careful about changing them ahead of time. I just went through this and was strongly advised by several in IT not to mess with it. You can't just "add" a day anymore. To do so requires them canceling your current tickets, refunding you, then reissuing new tickets, then relinking to your
MDE. If for some reason those new tickets don't get relinked properly by midnight the day of the change then all of your FP's will be gone. And yes it has happened more than a few times. They can try and fix it but if its over a holiday they'll be nothing to fix it with.
Here's what you do, when you get down there just make sure and go to GS's at any park while you still have at least one day left on your passes and they can add it for you. It will be the same price too. It is much easier once you're on premise for them to do this than on the phone ahead of time.
You can risk it if you want to but make darn sure you let IT/Tickets do it and not a normal cast member. The key is to hold the canceled tickets in suspension until the other ones are fully booked. If the same IT CM does the complete process, and they must be done one at a time, then you're probably ok. Plan on it taking some time and you'll still have to go back to MDE and relink. If for some reason they do an update and it doesn't relink properly, which they are doing constantly, then you could be out of luck the next morning when you wake up
A lot of the horrible issues and stories concerning NexGen/FP+ come from people making changes, such as these, to their MDE.