I own a website which has a high domain authority and it was ranked on the first page on Google. My website had to go through a complete overhaul like a completely new website structure and a re-design. I was assured by the web designer that it wouldn't affect our SEO and domain authority. I understand that we might lose a little bit of traffic because some of our pages from our website had to be deleted and this will affect the domain authority but 80% drop of organic traffic is devastating. Currently, we have moved to a new CMS called Drupal. Has this happened to anyone before? And if so, how do you deal with this issue? What should I do? Would appreciate any advice on how to solve this which I'm really losing sleep over.