I had installed Technical Preview 10140 in VirtualBox under Windows 7. Can I upgrade to the RTM version from this? Will Win10 activate? Is activation needed for VirtualBox Win10? Or do I need to do Win7->Daz->Win10 even in VirtualBox?
Why don't install a fresh version of Win 10 on VM? Then activate using tool like "Microsoft Toolkit"? (search the forum for that tool). Or, if u need to upgrade from already non genuine WIndows you don't need to activate again Windows 10. Then, Virtual Machine is not different from real machine, so, yes, you have to activated Windows unleass, as I said, you upgrade from already activate system.
Windows 10 activation inside a VirtualBox VM works exactly the same way as on a physical PC. So your question isn't really related to VirtualBox. If your Insider Preview build 10140 is currently in an activated state, then you can upgrade to Win 10 RTM and you will still be activated. After upgrading, you can even do a clean install inside the same VM and will still be activated as well. If you create a new VM, then you won't be activated inside that. You only need to do the Win 7 -> Daz -> Win 10 route if your build 10140 is not currently activated.
I had just installed 10140 by dowloading the ISAO, not signed up for insider preview, not logged in with MSA. So I think it will not be activated. Probably will take the Daz route on VM.