Sometimes it is necessary to move from one NNTP server to another. This happens very rarely, but perhaps you change jobs, or one server is very flaky and you want to use another.
Changing the server is pretty easy, right? You just change
gnus-select-method to point to the new server?
Wrong!
Article numbers are not (in any way) kept synchronized between different
NNTP servers, and the only way Gnus keeps track of what articles
you have read is by keeping track of article numbers. So when you
change gnus-select-method, your .newsrc file becomes
worthless.
You can use the M-x gnus-group-clear-data-on-native-groups command to clear out all data that you have on your native groups. Use with caution.
Clear the data from the current group only—nix out marks and the
list of read articles (gnus-group-clear-data).
After changing servers, you must move the cache hierarchy away,
since the cached articles will have wrong article numbers, which will
affect which articles Gnus thinks are read.
gnus-group-clear-data-on-native-groups will ask you if you want
to have it done automatically; for gnus-group-clear-data, you
can use M-x gnus-cache-move-cache (but beware, it will move the
cache for all groups).