well, plenty of places could have moved out of their box. many urban properties are quite narrow; i lived on one not even 2.5 metres wide, so every 3m box would be in use (and potentially then some) on that street. (i didn't have an address living there either, but w3w wasn't going to help that problem much. easier to give a recognisable landmark.)
However, what3words is only accurate to 3m, so this change may not have affected their system at all.