I was booked into IAD today on a Q fare, got to ATL, asked at the club if I could switch to DCA (the DCA flight I wanted to get onto was very nearly sold out from looking at the seatmap, literally just the usual two C+ seats and two Y seats blocked, everything else occupied). Agent at the club said "let me check something" did some typing and confirmed me on the flight (in M, with no fare difference collected). He said "we can do it because it's less than 100 miles". This wasn't a co-terminal SDC since I got rebooked into M instead of Q.

¯\_(ツ)_/¯ I'll take it