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The shell of a 26 and 26.5 are identical, you are correct. However the liners are slightly different to compensate for that 5cm size "difference." Really the liner of a 26 is thicker in certain spots then a 26.5. So I could buy a new liner or go have them shaped, but it's easier for me to just get new boots.
Now of course, this is very manufacture and model specific. The are some brands and models where a 26.5 and a 26 is straight up the same exact boot, but it makes the consumer feel better when they think they have a 'perfect' boot size. Now the high end manufactures and models will actually have differences in the footbeds and liners, but it's all useless anyway because if a person is going to spend that much money on boots, they're probably going to have them professionally fitted, which might even lead to a custom liner. But there is only so much the shell can stretch physically, you still need to maintain the structural strength in the material.
Moral of the story: I got 27's.
If you would like to continue the boot conversation and really test my knowledge, feel free to message me.