Here are a few common mistakes companies make when redesigning their websites:
Changing page names
How are your customers supposed to find you if you’ve moved URL and not told them! You run the risk of losing the equity of your old URL (Gallagher, 2015)
Complicated restructuring
Although your website will look new and probably still have all the same functionality as before, the way in which you’re site map is set up, shows Google where to rank your pages in relation to how important they seem to be to the page. If Google doesn’t think your page is important, down in the rankings you go, bye bye traffic!
via Could re-designing you’re website lead to less traffic and sales? | ik71’s blog.