I know this is an old post, but it is coming up on Google First Page. So I thought it might help someone else.  I had the same problem, as we often move existing websites over from other platforms. I found this module that will handle it for you. 

https://www.odoo.com/apps/modules/10.0/pr1_url_redirect/

  You can also do it with jQuery: http://www.opengest.solutions/forum/aide-1/question/odoo-comment-faire-une-redirection-de-page-301-ou-302-12  

Is there any .htaccess one could write rules not to loose all old urls?

werkzung RequestRedirect probably.

I am rewritting routes manually in the controller. But I wish I knew a better way.

    @http.route('/my-old-path-to-event/<path:oldpage>', type='http', auth="public", website=True)
    def redirect_to_event(self, oldpage):
        return werkzeug.utils.redirect("/event", 301)

What I would do if I had more time would be to add a field "oldpage_key" on the event model. Then in my controller, search if I kind find my oldpage withing my events.

I think that in order to write a module, you would need to allow configuration through the UI of the controllers.