Microsoft started Windows 10 upgrade program with a reservation process few months back. While over 110 million devices are running Windows 10 already, they are planning to push Windows 10 as an “Optional Update” in Windows Update for all Windows 7 and Windows 8.1 customers. Early next year, they are planning to again re-categorizing Windows 10 as a “Recommended Update”. Of course, you will be clearly prompted to choose whether or not to continue. If you don’t like Windows 10 after upgrade, you will have 31 days to roll back to your previous Windows version.
Regarding automatic updates on metered connections,
Windows 10 will not automatically download updates on a metered connection unless there is a security issue addressed within the update. In addition, Windows 10 contains a number of features for those on metered connections, including monitoring data usage by application and setting data usage quotas.
Microsoft is also making easier to upgrade to Windows 10 using offline media. They will update the Media Creation Tool which is used to create DVD ISOs or USB keys, to create a single image capable of upgrading any 32bit or 64 bit, Home or Pro, device. You can use this media to upgrade any number of Genuine PCs, and even do clean installs wherever you have a Windows license.