Hi
It depends on how different the OS is.
If you buy a Windows tablet that is running Windows 8 or 8.1, then I would assume that you could install Windows 9 on it, but if you have a tablet running Android or an Apple operating system you wouldn't.
There has been a lot of suggestion that Windows 9 upgrade will be available free to Windows 9 users but that hasn't been confirmed.
It appears that there will be a lot of new information available this coming Tuesday, the 30th, when Microsoft announces the existence of Windows 9 or whatever it's going to be called officially.
It's expected that the Beta or Test version will be released to the public for testing at about the same time.
Mike