Definitely reply to old topics - that's one of the
many advantages of a forum like rootschat, and often how cousins find each other.
I agree that it is good to keep the names on the public forum rather than reply via personal message. After all, if everyone replied via PM you would never have found your great grandparent's names by googling, and rootschat often appears near the top of google searches.
However contacting someone via PM can also be useful to alert them to a reply on their thread as not everyone has notifications of replies enalbed, or sometimes replies can be missed especially if you receive a lot of them.
