There is no such thing as "Sub Posts" in wordpress already built in.

But it could be possible that you create a plugin that is introducing "Sub Posts" in the style you describe them.

Technically you're not talking about subposts but about the URL-Layout. So in wordpress you add an endpoint ("changelog") that you can handle with some plugin, for example switching display to some other post.

Once this did not properly work with endpoints but I think it's somehow fixed now. Haven't used it tough, so my answer is only informative so far.

###Related:
 * [Rewrite API][1]
 * [Ticket #12779 - Better support for custom post types in WP_Rewrite][2]
 * [Ticket #12605 - Unable to add Endpoints to custom post_types][3]
 * [Ticket #9476 - add_rewrite_endpoint() doesn't work][4]
 * [Ticket #2433 - Provide some API for WP_Rewrite][5] 


  [1]: http://codex.wordpress.org/Rewrite_API
  [2]: http://core.trac.wordpress.org/ticket/12779
  [3]: http://core.trac.wordpress.org/ticket/12605
  [4]: http://core.trac.wordpress.org/ticket/9476
  [5]: http://core.trac.wordpress.org/ticket/2433