*Does this mean that it's not safe to follow the Terminal command advice of Zerwas?*
I wouldn't because his advice will change the ownership on all files/folders properly owned by
unknown, a legal OS X user. Now, if you limit the changes to /Users/, that'll only change those inside all user subfolders and, IIRC, none of those should be owned by
unknown.
Since you migrated from a PPC to an ICBM (Intel-chip-based Mac), you should review
http://discussions.apple.com/thread.jspa?messageID=1872713