If you are going to use a new news server, you should never update the old server data, as each news server uses its own numbering for the articles.
So what you need to do is configure a brand new server in the Preferences-Servers and remove the old one.
After this you also need to delete the old server (and the newsgroups) in the Subscriptions panel. This can be done by means of the right mouse button above the news server and choosing the context menu item "Delete". See image:
After this you will have to subscribe to the newsgroups again using the plus (+) button at the bottom.
Make sure to select the new configured news server for the subscriptions: