The existing site is a WordPress site, is the replacement also being built in WP?
If it is then I would suggest developing it in a subdomain or separate domain and then using a backup/migration plugin (I like WPVivid) to migrate everything from the new version to the old one.
Thank you very much.
The original site is empty, but I wont point DNS to it, because there is a actual site in another server.
So I will create a sub domain and make a new site.
I use All In One WP Migration, because I have a perpetual license.