How to transfer a new site to an old domain without losing SEO?

Question

CMS

Hello, I raised the topic earlier with moving from the old site to the new one, but the task is a little different, we want to change the old site to a new one, the old one works for us on modx evo, the new one on open cart 2, we want to leave the old domain on which the old site is now , but replace with a new site and how to save all links in the search so that it does not greatly affect the search results, or so that it is slightly reflected, I understand that the architectures of the sites are different, but we needed to make a new site because the old one was a nightmare in all meanings, but on open cart 2 I like how everything is arranged and works

Answer 1

In general, it depends on the site, the number of pages in the index, a lot of things.
1. Save on the new site - old urls. this is a very big part of the success.
2. Save more or less content from the old structure (it's 100% impossible, which is understandable)
3. If possible, all h1, titles, meta tags should be saved as on the old site.
4. You need to understand that your linking will still be a little different, you will lose something and gain something.
this is, in short, you can paint each item for several technical specifications, but this is the most basic.
well, 100% that you will lose some traffic. I have never seen a harmless SEO move from one engine to another.

Answer 2

Transfer all texts and SEO elements from the old to
the new