You’ll have some process of data migration and validation, so you can setup a staging WordPress environment somewhere on the same server (like staging.recepti-kuvar.rs). You can do the following:
- Create the subdomain and protect it with htpasswd;
- Create a copy of the living site, using Duplicator plugin (or any other that you find easier);
- Restore the duplicator package in the subdomain directory;
- Install the new theme at the staging environment and fiddle with it, moving the shortcodes and everything else from the old theme, that you’ll need in order to have your content look good.
- Switch the post type of your recepti to posts (you can do this with SQL in phpMyAdmin or through some plugins);
- Once you have done all this and the staging looks and acts as the live site should – move the theme with the new code (which should be either in helper plugins or in a child theme) to the live site and switch the type of the posts the same way you did in the staging.
P.S. This is not the best process for switching a theme of a living site, but it’s more of a direction that you can follow. If you don’t have that many new posts every day, you can calmly work on the staging till everything is ready, them move to staging the newly registered users and posts that were made after you created staging and then use duplicator again for getting the staging back to the main domain.
NB! Be always sure to have a recent backup of your live site and an easy way to restore it!