No, you will not lose your SEO rankings. Any knowledgeable SEO company will take basic steps to ensure that your SEO rankings will not be lost. Here are some simple steps you can take. 1) Using Google's "site:" option, find out which pages are already in Google search. 2) Make a list of all URLs 3) Study the H1s, Meta tags, and content of each page. 4) Create a mapping file of all current URLs 5) Optimize each URL to new URLs - Sometime you will need to shorten long URLs. 6) Add 301 redirects in the new website - so all old URLs are captured 7) Make sure that you update nameservers and other domain settings to new web-server. 8) You may have to account for https forwarding URLs. Once the new site is live, adding Google analytics, search, tags should be completed. Once the new website goes live, there may be 2-7 days of time frame, where you may see some rankings as "missing" from results. This is normal, as the Google search engine's algorithm's are adjusting to the new website's structure and content. If you have followed the above steps, and some other best practices, you should not see a decrease in SEO rankings. 100% of our customers see an improvement in the SEO rankings. However it is not a simple "one step" or a switch. It is a process that will take time. From a few months to many months. Here is a very good video from Google's staff.