Windows 8.1 and Windows 7 users may be curious to know, which version of Windows 10 they would be upgraded to, after they avail of the free Windows 10 upgrade offer.

Microsoft has clarified this and this is how the upgrade will work:

When you upgrade, you’ll stay on like-to-like editions of Windows. For example, Windows 7 Home Premium will upgrade to Windows 10 Home. This is how it will work!

32-bit gets upgraded to 32-bt, and 64-bit to 64-bit. Do read this post on Windows 10 Upgrade Path & Matrix too.