A USB disk with hdd-usb image written on it creates a FAT32 partition in which /live folder contains filesystem.squashfs file (and others). So I'm overwriting one file with its newer version extracted from newer hdd-usb image. No syslinux problems, no re-config needed, it's the same file, just newer.
It seems to me that you intend to use your system as a
regular debian installation.
Well, almost. I'm quite new to Linux and the aim is to play, try things, and learn. First I tried VirtualBox on Windows with Ubuntu inside, didn't like it, so I switched to Debian. Next step was to try Debian on real hardware, hence the live-USB idea (the USB disk is also a great recovery tool, I have other tools there, like UBCD4Win). Probably next step will be a dual-boot PC, but not yet. Debian-live boots surprisingly fast from my USB flash disk.