In November of 2014, Microsoft said that all Lumia devices sporting Windows Phone 8.1 would be able to make the jump to Windows 10 Mobile (not its name at the time). A little under 18 months later, and it turns out that not all devices will be able to upgrade. 

The Windows Insider Program has been answering user questions on Twitter regarding the upgrade status for several devices. As far as the exceptions go, regarding Windows 10 Mobile upgrades, Microsoft cites the fact that along with specification requirements, the quality of user experiences was a contributing factor. As such, there are a few devices that remain absent from Microsoft's compatible upgrade list. 

The full list of devices are:

• Lumia 1520
• Lumia 930 
• Lumia 640 
• Lumia 640XL 
• Lumia 730
• Lumia 735
• Lumia 830
• Lumia 532
• Lumia 535
• Lumia 540
• Lumia 635 (1GB)
• Lumia 636 (1GB) 
• Lumia 638 (1GB)
• Lumia 430
• Lumia 435

Furthermore, Microsoft notes that the above list is it. There will be no second wave of upgrades. To that end, if you're current Windows Phone 8.1 device is not featured here, the chances of an impending Windows 10 Mobile update are very slim indeed.