Hi Clare,
I have never actually experienced something like this myself during previous visits to the Walt Disney World Resort but would expect that any Guests with Advance Dining Reservations already booked at a restaurant which is going to close for refurbishment, would be informed as soon as the refurbishment dates have been confirmed.
An article was published on the Orlando Sentinel on 3 February 2016 (http://www.orlandosentinel.com/business/brinkmann-on-business/os-disney-springs-fulton-layoffs-post.html) which suggests that the restaurant will close for renovations from early April with plans to reopen in late November.
I have absolutely no idea how much truth there is in that statement at all and would only take it as red once it has been officially confirmed on the Walt Disney World website.
At this moment in time, it is still possible to make Advance Dining reservations at
Fulton's Crab House right up to and including 3 September 2016
.
If you are anxious at all, I would recommend making an alternative Advance Dining Reservation for your anniversary meal now. You can always cancel one nearer the time.
Wishing you a magical holiday filled with memories which will last a lifetime!
Emma