Some context: I have been tasked with repairing a PC which is running non-genuine Windows 7 Professional. The PC does come with an OEM license for Windows 7 Home Premium, and I will be offering to help the owner go genuine by reinstalling this version. But there are a lot of personal files stored on the PC, and it is preferable that I leave these files in place rather than backing them up and restoring them afterward, because of reasons.
If I install Windows 7 Home Premium from disc, can I do so non-destructively i.e. without erasing the existing user account and information?
(This question is similar to some others, but those I can find seem to be either about upgrading rather than downgrading, or about downgrading without a reinstall, as opposed to reinstalling without erasing the target disk first.)