Moving WordPress Blog to Main Domain That Doesn’t Have WordPress Installed

I am asking this question on behalf of a client. Their blog is a wordpress blog and is on a subdomain and their main domain does not have wordpress installed. They want to move their subdomain wordpress blog to their main domain but they are worried about security. I have seen other questions similar to this but nothing exactly the same.

My question is are the security concerns valid and how can we cover those bases?