The associated thread settings would be based on the thread settings for the target category. If there's no forum specified for the target category, the thread will be removed. (Similarly, the thread would be moved if necessary if a different forum were specified.)
Fields are also associated with a category, so any fields that don't apply to the target category will be removed.
It would be better to have some kind of warning that some fields will be deleted when moving a resource to a specific category.
Moderators may move resources and delete fields inadvertently.