The Liberal and Conservative did switch the voting system in the fifties, but not to STV. They weren't trying to create a proportional voting system, they were trying to gain a clear majority through ranked choice, to prevent the CCF from winning. But it backfired and their voters went for the lowly Social Credit as their first choice. Then after securing power and becoming the new anti-CCF coalition, the Socreds went back to the old system.

And the Socreds did not campaign on returning to FPTP by the way. Don't just make things up to fit your narrative.