When changing a forum that has the forum_flag FORUM_FLAG_ACTIVE_TOPICS set (or not set) from 'category' to 'forum' or vice versa the flag is applied to the new forum type.
The default configuration when creating a new forum however is:
New 'forum': FORUM_FLAG_ACTIVE_TOPICS is set ("Display active topics: Yes")
New 'category': FORUM_FLAG_ACTIVE_TOPICS is not set ("Enable active topics: No")
Background: FORUM_FLAG_ACTIVE_TOPICS is used by categories and forums, but in different ways.