Proper group ID (gid) for Lion?
I haven't seen any difference in how OS X treats the primary group vs. a user's other groups, so I don't think it makes any real difference.  I'd be inclined to "fix" it for neatness, though.  These commands should set the primary group to 20, and then add a "secondary" membership in the old andrew group:

sudo dscl . create /Users/andrew PrimaryGroupID 20
sudo dseditgroup -o edit -a andrew -t user andrew


(Note: using dscl to add secondary group memberships doesn't quite do it right -- use dseditgroup instead.)