wildbill7145;393084 wroteI believe the installation process overwrites the previous operating system.
Correct, that's the way I understand it as well.. I haven't done an upgrade from a previous install as yet although I am posting this from a Windows 10 laptop (clean install however). I have also installed Win 10 twice previously but always clean install. Will certainly post more here as I learn.
As far as space used, 8 is approx. the same as 7 and I expect 10 to be only marginally bigger. Can't say that for sure though.