The Minimum hardware requirements for Windows 10 Mobile may hold the key to the question that whether devices like Lumia 530 with 4 GB internal storage would be able to upgrade to Windows 10 Mobile or not.
The requirement documentation clearly mentions that Windows 10 Mobile running devices with 4 GB internal storage are possible and “must include sufficient free space in the system and user partitions for future updates of OEM-owned components”. It also puts the condition that “to ensure successful upgrade, it is recommended that a device with 4 GB of nonremovable flash storage includes an SD card slot.”
So, it is quite clear that Lumia 530 with 4 GB internals storage and SD card slot meets the criteria quite well.
2.3.1 Flash type and layout
Devices that run Windows 10 Mobile must include a minimum of 4 GB of nonremovable flash storage, and they must implement a flash storage system using an e•MMC or UFS and optional SD card. See section 6.7.3 for additional requirements for the micro SD card slot.
2.3.2 Free up space for update
Devices that run Windows 10 Mobile must include sufficient free space in the system and user partitions for future updates of OEM-owned components. For more information, see the Windows 10 Partner Documentation when it is available.
2.3.3 User flash
Devices that run Windows 10 Mobile must include a user partition of size 1 GB or more of internal nonremovable e•MMC or UFS storage.
2.3.4 SD card
If an SD card is included in a device that runs Windows 10 Mobile, the SD card must support the SD Association’s SD 2.0 or SD 3.0 specifications (http://www.sdcard.org/). The SD 3.0 specification is backward compatible with SD 2.0.
Note: To ensure successful upgrade, it is recommended that a device with 4 GB of nonremovable flash storage includes an SD card slot.
