I believe this was the case on CO, and perhaps on UA as well - although you could only do the SDC equivalent into the same fare bucket, they would open up all fare buckets close to departure - in other words, if there were 5 seats in coach available, it would show 5 across all classes of service.
Delta might do something similar, except with a twist - open up last-minute room for SDC on some of the higher fares, but keep E & X (for example) zeroed out. I assume what they really want to get rid of his people buying the cheapest ticket of the day and then changing to the more expensive flight for free.