Our company is doing a website migration. (URL, design, content, platform). The old website has not been optimized for SEO purpose but is ranking well for the name of the company which is a general word. Now they have decided to 301 redirect all the pages and remove the old website after launching the new one. I have read lots of articles and all mentioned that you'd better not remove the old site for a while in order not to destroy SEO. Is it true? can you please let me know some reasons why we should keep the old site?