I have done to, as Mr N. Scott Pearson, and lose Windows 8.1 licence.
It is necessary to upgrade the firmware before, and this results in the deletion of windows keys in hardware. This loses licensed version (that you paid), but now it is no longer important because the upgrade to Windows X has to pay, since the deadline for a free upgrade expired. My advice: leave it as it is.
I'm asking these question because the company were I work would use this "stick" for a professional usage.
Probably the Win 10 Pro will be updated through an image. Anyway we (or our final customer) have the Win 10 Pro license keys.
We want only to be sure that the HW fully support the Pro version.
This message was posted on behalf of Intel Corporation
Based on this document Supported Operating Systems for Intel® Compute Stick, the majority of the Intel® Compute Stick models support the upgrade.