The seats are convertible because the size of the CE cabin varies with demand. CE could go back to row 10, or it could go back to row 4 or row 20. The seats are converted to be wider in CE and the curtain is moved accordingly.

Consequently on the seatmap there will be a "buffer zone" where you can't allocate seats - and this allows for the cabins to remain flexible subject to demand.

At T-72 the cabin sizes should be known and the full seat map will open up for selection. However there is still a danger than the curtain may move again and your seats moved again accordingly.

Plenty of information in the sticky about this.