Depends on where the actual install came from. If it is a valid install and just a pirated license key, changing the license key to what you purchase may work.
If the source of the actual installation is other than Microsoft, you really, really, really want to wipe and reinstall the whole thing.
You have zero idea what other goodies were included in that install. They frequently come with all sorts of malware built in.
If a family member came to me with this issue, there is one and only one thing I would do.
Full wipe and reinstall, no contest. No matter where the install came from.