Solution:
Using mod_rewrite you can set your .htaccess file to redirect the incoming links from http://example.com/2004/04/title_of_post.html
to http://example.com/2004/04/title-of-post/
by:
RewriteEngine on
RewriteRule ^([0-9]{4}/[0-9]{1,2}/.*)\.html $1/ [R=301,L]
There is also a plugin for wordpress, you may have a look link