While we were at WDW recently, I was able to change our PS with little notice. I'd hoped to get an earlier time for the PS or move it to a different night (I wanted to be able to go for the parade and Wishes at MK). Our original PS was for Tuesday at 7:35 and there were no earlier times for that night. We did get a PS for Monday but only had 7:20 available. I took it but would have preferred an earlier time.
So, it seems like if you are flexible on day and time, you can often get one close to the time you'll be eating there. If, though, you need an earlier time, it might be harder.
BTW - I was very pleased with the quality of the food we had. I'm not always too impressed with buffet food but the food here had good flavor and the pasta wasn't overcooked (common when it's cooked in large quantities and kept warm for awhile).