Microsoft's head of Windows Insider program, Gabriel Aul, has confirmed that Windows 10 will require a fresh install on Windows devices older than Windows 7.
Revealed in a reply from his Twitter account, Aul wrote that there won't be any full upgrade paths as these will be solely available for Windows 7 and Windows 8.1 users.
It makes sense given that some of them might be on older filesystems (e.g. FAT32). That's also assuming that Windows 10 will run on these systems, which although they may have enough resources (RAM/CPU) to power Microsoft's newest OS, may well run into compatibility issues with legacy components/peripherals (rather than software).
That said, you can upgrade Windows XP to Windows 10 (Technical Preview) and still keep some (if not all) of your applications and settings in place. You will need to adhere to three rules (same bit and language versions) and have access to older Windows ISOs.
Microsoft is set to reveal more details about Windows 10 at an event scheduled on January 21 at its campus in Redmond. There's even talk of an updated version of Office for Mobile being announced there.