According to the Polish site windowsmania this list of Lumias Windows Phones can be upgraded to Windows 10 mobile.
Lumia Model 430DS, 520, 540, 435, 530, 620, 630, 635, 640XL ? , 720, 735, 830, 1020, 920, 925, 1320, 1520, L930,
Windowsmania.pl got hands on a list of Qualcomm SOCs that are compatible with Windows 10 mobile.
According to the table, if your Lumia smartphone is equipped with one of the Qualcomm chipsets in the list , then it is eligible to a Windows 10 Mobile upgrade.
Windows 10 Mobile also supports Intel soFIA LTE chipset, indicating Microsoft might launch new smartphones next year with this chipset in.
The picture below (made in May 2015) shows in some way what Lumias are going to be upgraded to W10M:
Disclaimer: Above information have to be officcialy confirmed by Microsoft.