I added the dining plan to my Sept 8 reservation the evening of Sept 7. Had to cancel the original reservation and pay in full for the dining plan and the stay. Ate lunch on dining credits on Sept 8. Created some confusion at Magical Express. Guest Services on the phone thought the Magic Express reservation would automatically transfer, but it did not. I had the paperwork on everything and the cast member did find us in the system and our bags followed us without a hassle. (Our return Magical Express trip was on a separate reservation since it was a split stay on which we didn't add dining, so was unaffected.) It took a while for the refund on the first reservation to come through, but it eventually did. Had no effect on FP+