Hi Alissa,
Thank you for your great question! It is perfectly fine to arrive for an Advanced Dining Reservation and then notify the server you're participating in the Disney Dining Plan. The server will then assist you with eligible menu items. I've attached a link for you to reference of the participating
Dining locations and associated table-service credits. Be sure to take note of which locations will redeem (2) table-service credits, for example, Dinner Shows.
Also be aware guests cannot check-in to their Disney owned & operated Resort Hotel and purchase the plan at that time as the Dining Plan would need to be added before 48-hours prior to your arrival. Only certain vacation packages are eligible so I'd suggest contacting a Cast Member at 407-939-7211 for further assistance.
As I'm booking my upcoming Advanced Dining Reservations, I've found it quite convenient to link them to the My Disney Experience app on my smartphone. I can simply reference each reservation with confirmation numbers at the touch of a button. For guests on the Disney Dining Plan, this new feature will likely become an essential tool in remaining organized throughout their trip.
Visit us again, Alissa, with any follow up questions. We're happy to help!
Sherri