Uninstall core forum.module while keeping content type and taxonomy
The problem is that there is no simple thing like "content type ownership". There is module column in {node_type} table, but simply changing it to "node" (as set for handmade content types) might not be enough. Forum module implements node-related hooks, and integrity of forum data might be endangered if you will simply turn them off and change module in database.

Safer option would be to export data you need to some neutral format, like XML, and then use Feeds or similar module to import it again, to the content type created with your new needs in mind.