Perhaps the new 1C and 1D were in 1A and 1B until that was needed for the lady and her elderly parent. If the gate agent was not sure if you would make the flight the gate agent may have swapped 1A and 1B rather than moving them elsewhere on the plane
Why didn't he tell me this, then? He insisted that 1C cannot be booked online and that the pax in 1C/1D needed assistance. And if there were additional pax needing assistance in other rows, why were they not seated there in place of the able-bodied couple?