In about 50 TED segments over the past year or so I have only had 2 mainline substitutions (a DEN-LAS and a LAS-SFO). They are known to happen (and they just make my day!).

Are you selecting seats during online checkin for a flight today or tomorrow, then yes, it could be they have swapped a mainline 319 for a Ted 320. If you are selecting seats for a flight several days out, probably not. In my experience, the swaps happen the day of the flight and not much further out than that.