If you decide not to take advantage of the upgrade, Windows10 upgrade will be paid. You'll need to pay around $120 for Home edition and around $200 for Professional.
Even if you'd like to stay on Windows 7 for a bit longer but think that you might want to upgrade in the future, you can take some steps to get the free upgrade. The upgrade is specific to the device not to your MS account. Backup your data. Register and install the free upgrade on your current Windows 7 machine. Then revert back to the Windows 7 OS. this way Microsoft will allow you to perform the free upgrade on this machine at a later date.