Migrations.sol‘s upgrade() won't execute correctly

[yushih]

I'm not 100% about understanding the upgrade() method correctly so apologies in advance if I got this wrong. The upgrade() method looks problematic: suppose upgraded.setCompleted has the same restricted() modifier as "this" contract, then it should fail because msg.sender is the address of "this" contract which could not be upgraded.owner. So there is no way the upgrade.setCompleted call could be executed successfully.