Is it possible to change OS on HTC Windows 8s device?
Changing the OS is not officially supported.
It also doesn't seem to be unofficially supported at the moment. There is almost no XDA development activity on the 8X or any other Windows Phone 8 device for that matter, so while it might be possible in the future to install Android, Ubuntu, Windows 8 RT, etc, it doesn't look like it will happen any time soon.
As far as I know, this is not (currently?) possible, and might never be possible. Then again, never say never when software is involved. ;-)
For some background on why this is, have a look at this article on WPCentral. All this should make Windows Phone even more interesting for enterprise use, which I think it a good thing.