Here's what I found on switching to moderated.

"From unmoderated to moderated.
Changing a group from unmoderated to moderated is ³strongly discouraged²
which, in practice, means ³practically impossible. ²
- There are many News Service Providers who would not make the
change. Some people would see a moderated group, others with different
NSPs would see the moderated version.
- Such changes in the past have cultivated massive resentments.
Leaving the original group intact and creating a companion moderated
group allows group members to have a choice between the two different
kinds of newsgroups on the same topic.
- In all likelihood, the board won't even publish an RFD to change a
group from unmoderated to moderated. A proposal for such a change should
not be submitted unless the group finds a way to demonstrate virtual
unanimity. We strongly recommend that you instead spend your effort on
learning how to use filters or how to create a moderated companion to
the original group."

marcella