Whether for a site relaunch with a change of domain or switch to https, if the site structure is changed, some important steps need to be followed.
Pages to redirect
All pages with value should be redirected: those with existing traffic and/or those with backlinks pointing to them.
The selected pages should redirect to the closest equivalent page: either exact page or the most related category page if this page doesn’t exist anymore.
Only pages with little value (no links and no traffic) should be “Killed” ideally with a 410.
Clean up errors
Current internal 301s should be fixed as well as links to 404
Old urls already redirecting
Old pages already redirecting especially those with backlinks should redirect to the new page therefore modifying the old redirect to avoid redirect chains.
It should be Mobile friendly with Optimal Site structure and main navigation (main pages close to homepage in number of clicks). Simple urls structure (short) & web structure
It should use only https, http version redirects to https
It should use only www or non-www, redirect non use version to the selected one.
Use canonicals on the new site, pointing to the new site urls. Self referencing except for very similar /duplicate content.
Identify Duplicate content / very similar content (use canonicals or redirects)
Set up a custom 404 page
Create sitemaps for all new urls that have SEO value only. Link it from Search Console when sure that the site is clean and urls submitted in Sitemaps are the right ones.
Set up Search console and Google Analytics. Use Robots.txt if needed
Before the live redirects, test them on test server (non indexable nor crawlable pages or protected by password or blocked by Robots)
After the Redirects are live
Update all online profiles, social media included, email signatures, etc
Compare Analytics data (before and after) check for traffic differences and which pages los traffic if any
Check Rankings before / after
Monitor indexed page count
Contact main sites linking to the old site to ask them to change the links.
Crawl old site to check for redirects
Crawl new site for errors
If the site structure of the new site is too different from the old one(s), there is a risk that Google will decide that the new site is a different one and some rankings may drop.
Either way, there will be temporary loss of traffic (so around 4 to 6 weeks are necessary to have clear idea how the migration went on)