Year to year little things can change on a binding. Say they make the plastic base on the binding 1 or 2mm thicker in order to beef it up. So if you put that new taller piece with the older shorter piece you have taken what has been designed as a flat plane that you release across into a crooked plane. Once you change that you get more unpredictable release. Hope that helps.
more importantly a shop will never ever ever mount those for you.