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Sorry, that's not likely to change due to the printing on the hardware itself:

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Hi can you please let us DDJ-400 owners change the function of the fx deck selector. I would really like to have master in middle and so i can just quickly slide from one to another deck fx
Sorry, that's not likely to change due to the printing on the hardware itself:

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could you at least unlock the midi mappings for it please?
Sorry, that's not available to be re-mapped.