In a similar situation, but with a cancelled award travel reservation. Original reservation was 2 adults with 1 lap infant (80k points)

Tried using Change function from My Reservations to book another flight that displays as one-way 20k/traveler, but on the calculation screen, it's telling me it will actually req 120k points

On top of that, our kid is no longer a lap infant and will need his own seat now. Not looking forward to calling Rep to sort through this.