The Redmond firm has announced that its latest operating system will not be rolled out to older devices until "early 2016".
"This November we introduced Windows 10 to phones including brand new features such as Continuum and Universal Windows Apps with the introduction of the Lumia 950 and 950 XL," said Microsoft in a statement.
"The Windows 10 Mobile upgrade will begin rolling out early next year to select existing Windows 8 and 8.1 phones."
The computing giant is remaining tight-lipped about which phones will receive Windows 10, confirming only that those with less than 8GB of storage will not be supported.
That rules out the likes of the Lumia 530, but a variety of Nokia-branded handsets are expected to be eligible.
Microsoft ushered in Windows 10 Mobile with the launch of its latest flagship smartphones, the Lumia 950 and the Lumia 950 XL, last month.