Interesting to see this thread active recently. I have a reservation on an ERJ-175 and chose a single side First Seat at booking. Checked again today and had been moved to a D seat on the 2 seat side. Called to see what happened and was told it was an "operational change" and could be due to a plane swap. Route has been all 175s for a while now so I doubt that explanation. I asked if the could "operationally change" me back to 2D to no avail, not sure what happened.