I have two earlier versions of Kontakt 6, 7, and I want to upgrade to 8. Can I remove 6 and 7 so I'll have more disc space and not lose any instruments?
It depends on what DAW you're using. Only Logic and Cubase have an auto migration for older versions of Kontakt: Notes about Auto-Migration of Kontakt Versions in DAW Projects
If you don't use one of these DAWs, then you need to keep the versions of Kontakt you have used in projects if you want to open them and not loose the content.