In a statement released earlier in the month during the Windows 10 Technical Preview Yamaha recommended against upgrading to Windows 10 until compatibility could be confirmed after users reported problems with the Vocaloid 4 Editor on Microsoft’s new OS. For the moment Yamaha is regarding Windows 10 as an unsupported OS until compatibility can be confirmed.

While Windows 10 is exciting, new, and upgrade free for Windows 7 & 8 users it is recommended you hold off upgrading for at least a couple months as both Microsoft and software vendors sort out the release issues. This post will be updated as the situation changes. For now, it is recommended against upgrading.

For those of you whom jumped the gun and have already upgraded to Windows 10 fear not. As long as you did not delete the 15GB Windows.Old folder you can still revert back to your Windows 7/8.1 install by following this guide.

Source: Yamaha Windows 10 Statement (Japanese)