If you want to change an existing column you have to remove replication. If you want to add a column or drop a column SQL2000 has two store procedure sp_repladdcolumn and sp_repldropcolumn which allow you to so without removing the replication.
There are ways to do it, some easier than others depending on the version of SQL you have. If you look at the list of articles I've publishered there are two on the subject, for for SQL 7 and for SQL2K - title starts with "altering replicated". Look that over, if you've got questions after that I'll be glad to help.