WordPress, Drupal and eZ Publish Data and Content Migrations
Make sure to hire professionals who know both systems - the target and the source. Otherwise you might loose risking history data with crude migration methods and possibly search engine rankings on Google, Bing, etc. due to improper use of redirects and Canonical URLs.
Or simply running out of budget or missing an important deadline. An experienced vendor will reuse intermediary migration formats like WordPress eXtended Rss (WXR). An example of a WXR importer for Bolt CMS - in this case it makes trivial to transfer content into the CMS from WordPress.
Professionals with experience from Content Migrations also know to why and how to use CMS APIs in content instead of raw database queries that might lead to unexpected issues later on in the development and deployment process.
With the added introduction of common Symfony Components to Content Management Systems it's ever easier, but still a challenge to handle. If the company you choose is large enough and a diverse product portfolio then that is good grounds for a successful content migration project.
One such company that masters Symfony, eZ Platform, Drupal and WordPress is Exove. Another is Kaliop. I recommend contacting these companies for more information and reading another blog entry on Content Migrations on the eZ Systems blog.