Is Windows 10 upgrade available only to those who have reserved it? Can I upgrade it without reserving if I have genuine Windows 7?
No, it is available to anyone who is eligible for the upgrade. According to the Windows 10 FAQ:
Can I get my free Windows 10 upgrade even if I didn’t reserve?
Yes. The easiest way to get the free upgrade is to reserve, but you can upgrade even if you don’t reserve.
Once Windows 10 is available, we’ll begin notifying devices that their free upgrade is available. Just open the Get Windows 10 app to schedule your upgrade. Note: Some notifications will go out as soon as Windows 10 is available; others may go out in the weeks or months following.
What happens when I reserve?
When you reserve, you can confirm your device is compatible with Windows 10. Between reservation and when your upgrade is ready, the files you need for the upgrade will be downloaded to your PC to make the final installation go more quickly. Then, when your upgrade is ready after July 29, 2015, you get a notification that lets you get started with your upgrade.
Reserving just allows your system to start making preparations and will help the upgrade process run smoother when it is released.