WordPress MU

hi! i am changing from simple wordpress to Wordpress MU. I exported my blog file and imported on WP Mu but i forgot to tell my authors to register at the new site so all posts were attributed to "admin". Now that i have my authors registered, i want to import again (from a newer *.xml file) attributing the posts to their respective authors.
But my question is, Will the posts and pages from this new *.xml file replace the current ones (as i want) or will they add to the ones existing (what i don't want to happen)?
Thanks for your replies!

Maybe it's wise to delete 'the current ones' first and then import again?

Thanks!
I thought that but it might become tedious so that's why i asked if importing again will replace the current posts, so i will save the process of deleting the old posts.
Thanks!