I'd like to change one category of our forum to be accessible only to registered user. This category actually contains about 30-40% of all the forum's urls. Google won't be able to crawl these pages anymore and will be redirected to the login/register page.

is there seo-wise a problem to do that? i mean would that affect ranking (duplicate content) or what ever?

has anybody some hints or experience with that?

cheers,
cesare

Content loss is never something that Search engines will appreciate.

If I where you, I would probably open a new forum for the new usage and leave the old one as it was, even locked for posting if required.