I have built a store using 3.0.15 in a sub directory and I now want to go live. So I thought I might as well upgrade at the same time to 3.0.17 in the root directory. Once this is loaded will I be able to put a holding page using an index.html page while I get the new store in order? Or will there be a conflict between the 2 index pages? What are the major headaches in doing this?

Firstly i would highly recommend that you do not use a holding page as it detracts from the purpose of your website and adds extra unneccessary clicks for the end user.

You can simply more the entire contents of your development store by using FileManager or its equivilent from your hosting control panel and then edit /includes/global.inc.php to change the store paths to their new root location. Then all you need to do is follow the upgrade instructions for each version.

If this isnt something you are comfortable doing then you will find many professionals at http://www.cubecartforums.org who can do it for you for a small fee.

Good luck