 :The length of the meter is defined in terms of the speed of light and the second so it could, even in principle, change as ambiguities in the definitions are clarified.  In addition, the realisation of the standard metre could indeed change every year as newer methods realisation are found.  :This is not that different from how things were when there was a standard metre rod when you get down to the fine detail.  For example, the engraved lines on the Paris standard will have finite width so a decision would have to be made as to where within the engraved line the end points were.  Just stating 'the centre', for example, would not be sufficient without stating exactly how the centre was to be defined. When that has been done, there is still the question of experimental uncertainties in the actual realisation.  :As it is now, the uncertainties in realising the metre using the current light-based standard are much less than they would be if the old metal bar standard were used.   