Which Lumia Smartphones are upgradable to Windows 10 Mobile?

Microsoft has announced that all Windows 8 or 8.1 phones will be upgraded to Windows 10. However, this statement also comes with a twist which states that even if the phone is running on Windows 8 or 8.1, it may not be upgraded to Windows 10 if the hardware does not support it. The statement comes with a line "Today's Lumia phones", which could mean a lot of things. More of a possibility shows that the Lumia phones which are more than two year old, but running on Windows 8/8.1 may not end up getting Windows 10.
Most probable list can include Lumia 435, Lumia 735 and Lumia 930.
The update could be supported on the following devices -
1. Lumia 520
2. Lumia 630
3. Lumia 635
4. Lumia 530
5. Lumia 1020
6. Lumia 1520
7. Lumia 830
8. Lumia 930
9. Lumia 1320
10.Lumia 526
11. Lumia 620
12. Lumia 636
13. Lumia 638
14. Lumia 720
15. Lumia 730
16.Lumia 810
17.Lumia 820
18. Lumia 822
19. Lumia 435
20. Lumia 920
21. Lumia 925
22. Lumia 928
23. Lumia ICON
24. Lumia 430

Please note that this list is not supported by any confirmation from Microsoft. This is based on what Microsoft has stated about the update process. Some of the devices in this list may get omitted or some newer ones may be added.
Incidentally, Microsoft has also come on record that all Windows 8/8.1 devices with at least 512MB RAM are proposed to be upgraded, but with varied functionality.

If you have a Lumia smartphone which comes pack in any of the listed Qualcomm SoCs, then you can get the Windows 10 mobile update for it.

The table also suggests that the popular Lumia 520 smartphone is all set to get the benefit from the update of Windows 10 as the Qualcomm chipset MSM8227 is listed as "SoCs supported in the Windows smartphones which can be updated to the trending/latest Windows 10." The Lumia 920 (MSM8960) and the Lumia 830 (MSM8926) are also in the line to get the same update. Some of the other Lumia phones such as Lumia 535, Lumia 630, Lumia 635 and Lumia 735 are also going to get the Windwos 10 update.

In November 2014, Lumia's official twitter handle confirmed that all the Lumia smartphones working on the Windows 8 will surely get upgraded to the latest Windows 10.

The given below is the complete list of all the Microsoft Lumia smartphones which are confirmed to get the Windows 10 mobile upgrade:

1. Lumia 430

2. Lumia 435

3. Lumia 532

4. Lumia 535

5. Lumia 540

6. Lumia 640

7. Lumia 640 XL

8. Lumia 735

9. Lumia 830

10. Lumia 930

So if you have your Lumia smartphone included in this list then you will surely have the Windows 10 mobile update for your Lumia handset.