This will be the fourth year in a row that I've gone down there either the first or second week of May. I have seen walkups turned away every trip. Saying that, I am also one of the ones who decided to try Sci Fi as a walk up a couple of years ago. I was told no walkups were being taken, so stepped back, called WDW Dining on my cell phone, and was given an ADR for just 20 minutes later. As I walked back up to the podium the CM recognized me, so looked a little confused. I told him I had an ADR now and wriggled my cell phone at him. As I gave him my name and he looked again at his screen, laughed, and said 'neat trick'. That's been the only time I've ever been able to do that though. I always keep the WDW dine number in my cell phone, just in case I decide to change my ADRs while I'm down there.
I'm on the DxDDP for my trip May 3rd-10th, so made my ADRs at 180 days out. I've tried several times to change two of my ADRs, but have not been successful. I'd call now for ADRs if you can. Sometimes the dining CMs will help you out. I'm going solo and I know when I called, on two different ADRs when I asked for a table for one, he said "I can seat the three of you at _____ time. When I corrected him and said again I was solo, he laughed, and again said, "As I said, I can seat the three of you at _______time." I laughed and said 'I guess my dining partners are going to be sick that day?' He again laughed and said "either that or such these are such popular restaurants, you may always pick up someone in the park that day".