Migrating Joomla to WordPress

[ Moderator note: moved to How-to and Troubleshooting. ]

Hey,

I just started working as Web-developer, working with WP, so I´m pretty new and I hope I´m in the right Forum.

My customer uses Joomla and i rebuild his site to WordPress.

The customer is very well placed on Google, so ofcourse I´m afraid, that he will drop. What should I consider when i migrate him to the new CMS?

So far I was only looking for the URL-structure example:
http://www.site.de/news/post1.HTML stays ALMOST the same on the new site.

As you may guessed, my new side has no .html in the end of the URL like the existing does. Is this a problem? Will the site drop cause of this?

What other stuff should I think about, considering SEO?

I appreciate any help and also any tipps considering SEO at all 🙂

Thx in advance
Mosl