They need to change their constitution after Trudeau.
That’s not how it works. The Governor General of Canada, on behalf of the King, is required to appoint as Prime Minister whomever can command a majority of the House of Commons. There are no age or term restrictions, the Prime Minister must simply be a member of the House of Commons and is usually the leader of the largest party or a coalition of parties.
General Elections must be held every five years. Given current polling, the Liberals are going to lose the next election very badly. The GG will appoint Pierre Polilievre as PM and Trudeau will probably not hang around as Leader of the Opposition for very long after the election.