MS Office trial: Purchase & activation question.

Sorry if this has been asked & answered elsewhere, but searching didn't reveal comment on this particular issue.

I just got an XPS 8930 which has Microsoft Office pre loaded (or seems to have, anyway).

Selecting any of the applications from the start menu takes me to an option to purchase or activate the software.  My question is, if I continue the process to purchase, will it initiate a new download & install, or will it just activate what's already installed?  If it does a new download/install, should I uninstall the existing software first?

This is a nice fresh machine and I'd rather not crud it up with multiple copies and/or trial stuff.  I also need to be mindful of space considerations since I boot from a SSD that also hosts the program files.

Any advice?