Export / Import on wordpress.com hosted

I’ve raised this question as an addendum on another topic, but I’d just like to raise it in it’s own right …

I am trying to clean up some data on a blog hosted on wordpress.com

Since I can’t amend the DB directly I have exported the data to another DB and done the work. Now I have created a new export file with the clean data.

My question is whether it’s safe just to import that back to the wordpress.com hosted site. I assume that I have to trash the existing comments and posts first otherwise it will double them up. Is that right?

If so is there a way to backup the data before I do this. I can’t see the backup option like I can on my regular wordpress sites.

Thanks