I don't think you can able to do that since the upgrade firmware on the WebGUI is for upgrading the unit to latest firmware downgrading won't work in the web interface.
If you upgrade it with different firmware or a lower version you might get some error like "Can't find signature" You can only bypass this error if you flash the control board through an SD card/OTG.
I actually remembered it differently, so I've tried it out a few times now and I have to agree with you. There is no general solution for this as it does not work with every firmware.
To be sure, I tried the web interface process again with different firmwares on an S19. In some cases it worked, but in most cases the message came:
"Cant find signature"That probably just serves as a security measure to prevent the installation of unauthorized or incompatible firmware. So the only way is the way with SD card as @BitMaxz already mentioned.