This is the first time I've heard that rumor, and I doubt they'd take it down for a rehab in this busy summer season. Especially not if the rumor is true that they might sell premium seating for World of Color in the outdoor section of Ariel's Grotto restaurant!
The rehab rumor would go together with the rumor I did hear of Ariel doing meet-and-greets by her new ride (when it opens), if that rumor is true. It doesn't make that much sense for DCA to have two significant spaces devoted to Ariel, and it would make absolutely no sense for them to have two meet-and-greet spaces. (Though I only heard the meet-and-greet rumor once so that may not be true, fair enough.

)
Anyway, I really think you're fine but as a planner, if I were you, I'd just think of a back-up reservation you'd make on the off chance that Ariel's isn't open (such as Goofy's Kitchen or a breakfast, or even a regular Table Service restaurant). Then you don't have to worry because you're even prepared for the unexpected.
Have a great trip!