Hey John,
I don't know for certain if it will write over the previously installed version or not but I always uninstall then install the one I want. Be sure to back up your XML file and screen set if it is not the standard. I keep a MachC folder on my desktop. In it you will find my XML files, screen sets, licenses files. I keep these up to date so if something did go wrong with a download, I'd have it all handy and backed up. Nothing has went wrong on a download yet, but just to be safe.
Brett