How to replace one site with another?

Question

Search Engine Optimization

Good day.
From the previous owners of the business, they got a site that does not fit the current requirements in any way.
But it was brought to the tops of Yandex and Google search!
The new site has been written and is ready to be launched, so the question arose: how to replace one site with another so that it does not sink in the search engines?

Answer 1

If the content is different, it will sag.
If technically different, on the same content, then it is important that the old URLs are preserved (this can be done by setting up redirects on the web server).

Answer 2

Hiring an SEO specialist who organizes everything, without preparation, is likely to sag a lot.