In the Beta release 10.12, Sierra Boot Camp Assistant does not allow creation of Boot Camp partitions with any version of Windows older than v10.
This contravenes many Apple advice pages on the internet such as: https://support.apple.com/en-au/HT201457 Use Windows 8.1 on your Mac with Boot Camp.
I do not know what would happen to anyone with an existing Win8.1 BootCamp partition if they upgraded to Sierra.
Hopefully, this betrayal of public commitment will be fixed in the final release.
This contravenes many Apple advice pages on the internet such as: https://support.apple.com/en-au/HT201457 Use Windows 8.1 on your Mac with Boot Camp.
I do not know what would happen to anyone with an existing Win8.1 BootCamp partition if they upgraded to Sierra.
Hopefully, this betrayal of public commitment will be fixed in the final release.