There are two sites on the issue with the priority of the first one, what is the best way to do it?

Question

Nginx

A new site and an old one, the client wants to make a knight's move.
It is necessary that the new site opens and works by default, if a person moves from the search using the old structure (and it has changed by 98%), then he sees the old site.
I think tomorrow I will try to implement it in nignx + apache
I think so
upstream to two backends, one on 127.0.0.0
, the second on the ip of the old site.
Split via try_files
Logic in priority is the new site, respectively, the new site is given first, which gives 404 hence the old site.
Can someone tell me how best to do it
If a piece of the config throws a special thank you