If you have edited your local.conf so it sets MACHINE with = rather than the default ?=, the automatic upgrade helper doesn't know this and assumes that setting MACHINE in the external environment will still switch the machine, which it won't.
Move to 2.2 due to my current load in QA transition.
Patches posted to yocto@ list and available here: http://git.yoctoproject.org/cgit/cgit.cgi/auto-upgrade-helper/log/?h=devel