Here the FAQ from Microsoft:
Q: Are all phones eligible for the Windows 10 Mobile upgrade?
A: To ensure the best experience possible for our Windows Phones customers, we are upgrading devices which will offer a great Windows 10 Mobile experience after the upgrade to Windows 10.
Q: How will the upgrade work?
A: You can check on upgrade eligibility to Windows 10 Mobile by downloading the Upgrade Advisor app from the Store on Windows Phone. On eligible phones, the app will provide and option to Enable Windows 10 upgrade. Once enabled, the update can be downloaded by going to Settings > Phone Updates > Check for updates.
Q: I checked the Upgrade Advisor app and it said my phone was not eligible for the upgrade, how do I get Windows 10 Mobile?
A: To ensure the best Windows 10 Mobile experience possible, only select Windows Phone devices are able to upgrade to Windows 10 Mobile. The availability of Windows 10 Mobile as an upgrade for existing Windows Phone 8.1 devices will vary by device manufacturer, device model, country or region, mobile operator or service provider, hardware limitations, and other factors.
Q: If I receive a notification from Upgrade Advisor that the Windows 10 update is available for my phone, do I have to update?
A: No, you can stick with your current version of Windows Phone 8.1. Simply do not select “Enable Windows 10 upgrade” within the app.
Q: I’m a Windows Insider, what should I do if I want the ‘official’ Windows 10Mobile upgrade?
A: As a Windows Insider please read
https://answers.microsoft.com/en-us...-fe98-482e-850d-7ca86433bd7d?tm=1458232411654 to get answers for common questions regarding the upgrade. We also recommend visiting
Supported phones for Windows Insider Preview - Microsoft Windows to understand whether or not your phone is on the program’s supported device list, as this may impact your ability to get future preview updates. If you wish leave the Windows Insider program and check on your device’s eligibility for Windows 10 Mobile using the Upgrade Advisor app, you will need to first go back to Windows Phone 8.1. To go back to a previous version, see Windows Device Recovery Tool: FAQ.
Q: Can I go back to my previous version of Windows Phone 8.1?
If you upgraded your Windows Phone 8.1 device to Windows 10 Mobile, you can use the Windows Device Recovery Tool to rollback to a previous version. For more info about this tool, see Windows Device Recovery Tool: FAQ.
SOURCE: Microsoft
well in others words this really is something that Microsoft aint pushing down the throat of the windows phone users. as with the pc/laptop . here you have a choice to remain with the current windows phone 8.1 . it will be interesting to see by when msft start forcing the update as mandatory.
happy updating.