Fine to Make Large Scale Site Changes at Same Time But Cause Bigger SERP Fluctuations Than Phased Changes
No problem with making large scale changes to the content on a site all at once, but this will result in larger SERP fluctuations. Spreading content changes over time will result in smaller fluctuations over a longer period of time.
Adding Bulk Pages Is OK
Adding a large number of pages in bulk, such as adding an archive, is fine from an SEO point of view, however, Googlebot will try and crawl the new pages so helps if server has capacity to deal with that.
Use Canonical Tags to Test a New Website Before Migration
When you want to test a new website in parallel with the old one, you can launch the new site on a sub-domain and canonicalise the new pages to old pages.
Link Authority Is Lost When a Site Completely Changes
If the content on a site significantly changes to a new topic, any authority from backlinks will be lost.
Crawl Rate Increases After Site-Wide Changes
Google crawls site-wide changes a bit faster than they normally would.
Expect Ranking Fluctuations Following a Platform Migration
Google will take some time to reprocess a site following a platform change, especially if URLs and internal linking have changed, but if you see any long term impact you should explore any potential issues.
Google Detects if a Site Completely Changes Content
If you buy an old domain and change the content, Google will treat it like a new website.
Don’t Change URLs and Domains at the Same Time
Don’t change URL paths when changing domains or protocol, as it makes it harder for Google to process the change.
WWW Subdomain Should Not be "Parked"
If the www version of your domain is "parked", it might affect the indexing of the other versions of your domain.