No, not between SDHC, SDXC and SDUC cards, i.e. SD card specifications since version 2.0.
But there was a backward-incompatible change by redefining the card-specific data register (CSD register in the SD card) by the SD card specification 2.0 (versus v1.x, i.e. from SDSC to SDHC).
Sounds a lot like it will be a SW-only update. GPT vs MBR and some new commands.
Yes, on the host side the device driver for SD cards must support the new register layout for SDHC cards, though this change is rather analogue to the change in the ATA specifications (by the T13 committee) which altered the addressing scheme from CHS- to LBR-addressing in ATA harddisk-drives.
So, as long as solely the aspect “size” is addressed, “requires only software changes” is basically true for the host side and I assume the SD card association will adhere to this in the future. Consequently I shortened “smartphone hardware” twice to “smartphones” in my prior posting.
And while it is also true, that SD cards mimic an MMC card at startup (1 bit data at 20, later 25 / 26 MHz = 2,5 to 3,2 MiB/s), the host must command them into higher speed transfer modes, for some of which the electrical interface (i.e. the “physical layer (PHY)”) is radically different: 1,8 volts operation (originally 3,3 volts only) for UHS-I, -II and -III, LVDS (low voltage differential signalling) for UHS-II and -III which doubles the data pins from 4 to 8, and the SDexpress specification (introduced with SD card specification 7.0) which adds even more mandatory pins, redefines the pins for a PCIe physical layer and uses the NVMe protocol (i.e. an additional, completely different set of registers). All these features must be supported by the host hardware and software to utilise them.
Side note: I believe to remember (but am too lazy to look that up), that SDexpress cards do not need to support the original MMC mode (3.3 V, 20 MHz, 1 bit), but they must at least support UHS-I mode (1,8 V, 100 MHz, 4 bits). In the context of this discussion this would constitute a breaking hardware change, as most older host devices will probe an SD card in MMC mode (which then fails, but the host’s device driver does not retry in UHS-I mode) and very old host device hardware (>> 5 years) might not support UHS-I mode.
But imagine transferring the content of a 2 TB SDXC card via High Speed (HS) mode (3.3 V, 50 MHz, 4 bits), which may be the fastest mode an old host device designed for SDHC cards supports, i.e. with 25 MB/s: 2.000.000 / 25 s = 80.000 seconds = 22,2 hours. Hence it is not practically feasible to handle so much data on an SD card in an old device, IMO.
