Wireless carrier Vodafone Australia said that it is waiting for "bits" of the OS to test, and expects this to happen by the middle of this month. Another mobile operator, Movistar Spain, now expects to push out Windows 10 Mobile at the end of this month.
Tweets sent out today from Microsoft's Lumia Support Twitter account reveal that the Windows 10 Mobile update will be taking place "soon." If we put together the comments made by the carriers with the tweets disseminated by Microsoft, it would appear that the Windows 10 Mobile update will start making their way to Windows Phone 8.1 powered handsets before the middle of next month.
According to the Lumia support page, those handsets receiving Windows 10 Mobile must have at least 8GB of internal storage. In addition, "not every phone will support all possible Windows 10 features. Certain features and experiences will require more advanced future hardware." Only phones loaded with Lumia Denim (OS version 8.10.14219.341) can be updated. Go to Settings > extras+info to check the current software release on your phone. To see what OS version your phone is currently running, go to Settings > about > more information.
source: @LumiaHelp, Lumia via WMPoweruser