A strange phenomenon occurs in our system of government after elections. If an opposition party manages to unseat the government they have to switch sides. Our parliamentary tradition is purposefully antagonistic. Opposition parties are expected to criticize the government and challenge its policies. This structure results in some very unusual moments when the newly displaced governing party can begin to criticize policies they only weeks/months before they were responsible for.
There were some bizarre moments in the wake of the 2015 federal election where sharp critiques came from the Conservative opposition. The Conservative Party were a dangerous opponent to the inexperienced Liberal government because in many cases the critic knew more about the ministry than the minister answering.
Parties have to undergo significant transformations to make the transition successful. The internal culture, language and structures of a party are quite different between government and opposition. Oftentimes what makes a party succeed at one would cripple it in the other. The Conservatives have succeeded in their criticism in part because they have managed to hold on to the discipline the maintained in government. Outside of the House the party shreds itself over the leadership race, but interim leader Rona Ambrose seems to be doing a good job holding feet to the fire.
I think politicians and partisans must do a certain amount of double-think to pull this off. Somehow the issue they didn't voice any concern over years in power are a scandal on the opposition side. The move from government to opposition can reveal factions within the party that had been muzzled for the sake of party unity. Stephen Harper ran massive budget deficits during his tenure, but budget hawks kept quiet. Now they can release their bile on the Trudeau deficits. Unmanaged these factions can tear a party apart. However, the leadership race that accompanies this sort of switch is often a battle of the soul and who is best posed to challenge the new government.
As a New Democrat I am used to my party being on the opposition benches. There is a strange thing that happens with that party as well. As the target changes so does the rhetoric. The NDP can have a more difficult time finding a way to criticize centrist Liberals than right-wing Conservatives. The Harper government could be a crisis of democracy but Trudeau, well... This issue is particularly noticeable for Liberals if they have to switch from criticize the NDP to the Conservatives, or vice versa, as has happened in the provinces.
Much has been made of how the Trudeau Liberals and Harper Conservatives are increasingly resembling each other, especially as the Liberals move to curtail debate. Ultimately there seems to be patterns in our politics that often transcends party. These changes in position force a rethink of party positions and posturing. For the NDP, does the party make another bid for government? Do they aim to recapture Official Opposition status first? Or, does it return to being the permanent third (fourth/fifth) party? How they choose to challenge the Liberals and select as a leader will determine that result to a great extent.