I couldn't answer 'never'. 'when I feel like it' makes sense. Reality is, rarely do i switch out parts, as it means digging out the box and opening it, which as you know, can be a pain in the butt and a lot of work, fighting the vacuum
..if i had a way of storing all the spare heads, arms, out of the boxes in an easy access sponge lined case, then i would consider switching out parts more often. Maybe Sideshow could sell a funky sponge lined case that offers many ways to store statue parts.