I hadn't thought about chemo changing hair texture, and that would definitely explain Catherine's already-abundant hair looking even more full.

Catherine has dealt with a lot over the past two years. If she wants to change things up, more power to her. Besides which, she's not the first Royal to change hair color. Remember Madeleine of Sweden ditching her blonde color for a brunette look? She looked stunning either way. So does Catherine. You do what makes you happy.