NokiaPowerUser reports now that January 12 may be the final roll-out of the system upgrade and that Microsoft will also release the speculated Lumia 850
Microsoft may be able to push the update to devices without the carriers' involvement as a result to the improved update behavior introduced with Windows 10 Mobile.
Those running on Windows 10 Mobile preview builds already should definitely get it right away. However, phones running on Windows Phone 8.1 surely will still require carrier approval to upgrade to Windows 10 Mobile.
Microsoft´s transition to Windows 10 Mobile results to be complicated and it seems the company is trying hard to deliver a convincing product. Market share of Windows Phone is down to just 1.7% according to Gartner figures for Q3 of 2015. In Q3 of 2014, Windows Phones still had a share of 3%.
International Data Corporation (IDC) predicts that Windows Phone / Windows 10 Mobile will not grow its market share until 2019. The prediction is based on Microsoft's lack of OEM partner support. In 4 years, Windows Phone´s market share is expected to only grow from 2.2% to 2.3%.