Current problem
Currently for any existing or future growing communities wanting to change a channel to a forum their either delete everything or the put the old channel as read only in a "archived" section. This comes with the downside of either loosing all the previous posts or adding clutter to the channel list with all the old messages being in a separate location than the new posts.
Proposed change
Allow a channel to be converted to a forum/media channel with all the previous messages being put in a post within the forum.