Neallo, JLM, that is the difference between American and Canadian systems. After election, conservatives could have got rid of Harper, elected a new leader (say, Peter McKay) and he would have been the PM..
Or let us take really a journey into never never land. Let us suppose that after the election, Dion switched form Liberal to Conservatives. Then Conservatives elect Dion as their leader. Then Harper is out, Dion becomes the PM. This may sound totally wacky, but it would be perfectly constitutional and legal.
However, in USA, they elected Obama. Only his resignation will result in his not being the President, nothing else. Democrats may throw him out of the party, or he may voluntarily join the Republicans, may become independent, may be involved in scandal, nothing matters. People voted for Obama government and that is what they have for four years.
In Canada, they voted for an MP, nothing more.