Can you modify free dining ressie after 6/4?

[dsnymomof4]

We currently have reservations for Pop for free dining 8/25 - 9/1. However, we have yet to book our airfare and for those dates there are no reasonable rates out there. I am hoping something will go on sale, but if I look at dates before/after mine, there are flights available. So, I am wondering if I would have to change my reservation prior to 6/4 to still be eligible for free dining. Is a change in dates considered a new reservation? Any info would be appreciated! Thanks!