I've been doing some initial testing on the migration script you've provided, and here are my results so far:
1) As a baseline, I installed both the V6 HF and SF releases. For the HF release, after expanding the file system (I am using an 8GB, Class 10 SD card), I installed Squeezeslave, which operated OK through the on-board sound card. There was some stuttering in the sound initially, as I was changing the sound level through a web browser to LMS installed on my QNAP NAS. After a minute or so, that settled down. I'll attempt to reproduce this when I test again.
For the SF release, again after expanding the file system, the installation of Squeezeslave was refused, as it is not supported. The "experimental" version of Squeezeslave also refused to install. To check operation, I installed Squeezelite, which worked (again through the onboard sound card).
2) After establishing a baseline, I downloaded the latest releases of Rasbian (HF, 2012-12-16) And Debian Wheezy (SF, 2012-08-08). I did not apply updates directly.
3) After installing Raspbian and expanding the file system, I followed your posted commands. There was a slight hiccup as, after logging out and logging in as root, an initial script runs which changed the logged in ID back to pi. After noticing that, the script ran fine, as far as I can recall (see later). The system updated and installed the V6 code, after asking me whether I was running HF or SF. Squeezeslave installed ok and ran, though to my ears the sound quality was worse than with your vanilla V6 release. That could simply be my ears deceiving me, and I will check again.
4) Debian Wheezy showed the same hiccup as Raspbian when going through the initial setup - but that was no issue. The script ran and downloaded updates, but there were errors in the download I noticed (broken connection). This occurred several times in the script execution, and repeated when I re-ran from scratch. The errors were in the update part of the script. I don't have a record of them as the logging failed to work. I will try again and see what happens. However, after installing the V6 code, I tried to install Squeezeslave, assuming it would fail. But the script told me it had installed. After rebooting, Squeezeslave didn't start of course. I "uninstalled" it and installed Squeezelite. That did install, but after getting it to run, the sound (still through the onboard soundcard) was only a noisy "mush". I suspect that errors in the update may be to blame, but I will try the test again.
Major issues appear to be that Squeezeslave script tells the user it is installing on SF when it isn't, unlike the vanilla V6 script which tells the user it is not supported.
Plan now is to reproduce the tests and collect logs if possible.