You can't change the Site Definition for a site after the site has been
created.

However, to the extent that these sites were built using a different
Feature set you can activate and deactivate Features to accomplish the
same result.

I looked at the features and was hoping the meeting  recurrence was a
feature. It is not. I'm not sure what would happen if you turned
the Team Collab. feature off, and then back on.

So I took a manual approach.

The big job was making the document libraries series and cleaning up
the empty folders.

The small job was using SPD to remove the meeting recurrence
information web part.

I'm pretty sure the Team Collab feature just populates the Create page
with links that can be used to create lists, libraries, pages, and
sites. I don't think turning it off and back on will do anything to an
existing site.