Then you make those positions redundant (or in british english terminology, you 'let them go'). It's perfectly legal (in EU which doesn't have this at-will stuff) to say "We no longer need a Django programmer, since we're moving to Ruby on Rails now, we know you're a great django programmer, but we're letting you go". You don't need 'at-will employment' to have this advantage. There are some limitations, like you actually have to make the position redundent, you can't hire your brother as a django developer the next day. But that's the point, it balances out the power.