Alain:
This will update OpenWRT and I guess in the future this will also automatically (?) save any userfiles to the SD Card instead to the internal memory ?
I've not tried doing that, but I would be VERY surprised if that was the case.
There's updating the system software, and there's expanding the disk space (setting up a pivot root on the SD card.) Two very different operations. I believe what you are describing is simply updating the system software.