"what if you already have XE6 installed on W8.1 , which is able to auto updated to W10. will that be OK?"
There is no real reason why XE6 will not work on Windows 10. In the same way that Delphi 7 will normally work on Windows 7.
However Embarcadero can only support the certified platforms for each product. Therefore as versions of Delphi prior to XE8 have not been tested or certififed for use on Windows 10, we are unable to support any issues in regard to the installation, registration or use of the products on these platforms.