Thankfully that does not appear to be the case. According to Microsoft’s Gabriel Aul who heads the Insider program, the Lumia Denim is not a prerequisite for would-be Windows 10 upgraders. Aul confirmed this on his Twitter account in response to a user asking if the Lumia Denim update was necessary to get Windows 10 on his smartphone.
This means is that even if you have yet to get the Lumia Denim update, you can rest easy knowing that regardless of whether you receive it not, you will not be locked out from upgrading to Windows 10. The Technical Preview for Windows 10’s release is still up in the air at the moment, although it was recently teased that it could see a release later this week.
Microsoft had also confirmed the ability for users to roll back to Windows Phone 8/8.1 should the Windows 10 Technical Preview prove to be too buggy as a daily driver, or if the user feels that they still prefer to use Windows Phone 8.1, at least until a more complete version of Windows 10 has been released.