Buy the upgrade its 1/2 the price and its the same download to install.
LR 4.3 is the present version and will install without affecting your LR 1.1 version. You will get a prompt during the instalation if you wish to convert your LR 1.1 catalog, this process will make a copy of the existing catalog and convert it so it is compatable with LR 4.x. Your original catalog will remain in place.
The "read me notes" PDF file, that comes with the download package give all the info and details you need.