Thanks for the feedback everyone. My MiL is 80 and I thought a lie flat seat in CW for one leg would be a nice treat for her, and that my wife and I would be next to her for the other three flights as originally planned to take care of her.

When I made the upgrade I had NO idea that it would trigger a split booking and cause the issue with advance seat reservation. Inflexible systems are hardly my fault! As the old saying goes - no good deed goes unpunished!!