What happens if a user invokes with -x in place of -i initially ?  I guess stuff breaks half-way through the upgrade, which might leave an unrecoverable state.  The puzzle then is whether it's better to document "do not use a -x flag on the command-line yourselves, no matter what you may have seen in ps's output in the past, it is reserved for internal use" or to remain silent and tell anyone who complains after trying it "well don't do that, then", since we never mentioned it as an argument they could sensibly use.  Then again, I'm not so sure stuff breaks, as long as we unlink files from the old installation before unpacking the new one; any Unix, at least, should keep the (now deleted) old binary around, albeit as an anonymous "file", until our execution of it exits.