If the url structure is to change you should plan it carefully and do it slowly. First use canonical links on all useful pages on the old site to the equivalent pages on the new site. Monitor traffic to the old pages. Once traffic slows down to the old pages, write 301 redirects for all useful pages on the old site to the equivalent page on the new site. Once traffic disappears from the old site you can remove the old pages but leave the 301 redirects in place for as long as you can. Reach out to those linking to you and request a new link to the new page.