Just changing the site URL in the config will likely not update all of the internals to create a working dev site for you (unless the site is pretty bare-bones). You'll have problems with serialized data not showing and links within posts pointing to the old site.
It would be smarter to use a migration tool like Backup Buddy or Duplicator to create a complete copy of the site that can be re-deployed at a new location with a new URL. Doing this, you'll still have working links within posts, any custom menu links, etc. Using one of these will make launching your changes simpler as well. Just package it all up and re-deploy to your production site when you're finished.
If you don't want to spend the time downloading/uploading everything again, you can just migrate the database using something like WP Migrate DB. Install it on your production site, export a database with the new URL and import the migrated database to your dev via phpMyAdmin or similar. Any hard-coded links in your theme will still need to be updated and your .htaccess will need to be updated if you're installing in a sub-folder.
Change the RewriteBase line to:
And the line that redirects to your index.php to:
RewriteRule . /yourfolder/index.php [L]