5.1.0 doesn't change diags partition.
/var/local/system/locale and /var/local/system/tzVar aren't sourced anymore in upstart scripts, but rather parsed. data.tar.gz extraction step is removed from appropriate upstart script.
But there is something new. /var/local/system/fixup and /var/local/system/onetimefixup are checked for existence and (on success) executed. And if /var/local/system/onetimefixup had been executed, it will be deleted afterwards.