I'm not sure that part of the article is correct. I've been using my old sync account in Nightly builds (now up to version 31) and it's still working fine.

Edit: I just checked with a clean install, and there's a button that says "Using an older version of sync?" It brings you to this page https://support.mozilla.org/en-US/kb/how-to-update-to-the-ne... which basically says "unlink all your devices and re-link them." So I guess adding new devices to your old account doesn't work.