I know that Box in the Sellier style is stiff and rigid, and makes getting in and out of the bag somewhat tricky, but what about Box in Retourne? Can anyone share their experience?
No expert here but I can say that Box will certainly soften and become more pliable with age, especially if it is cared for properly. This is regardless of the style or bag it is made into. I have my grandmothers BBK32 retourne, and it is quite easy to use.
That being said, a bag in sellier will always be a bit more stiff, even with age, because of the construction. I don't have any experience with a K32 sellier in box. but I would think that an older, well cared for one should not be tremendously stiff.