It was interesting to listen and chat about this at the QE2 Gathering on board Queen Victoria last month.
Several of the senior officers remembered 'Niagara' as the code, they also knew it had been changed...and a few had forgotten the new code!...
However there was a collective, 'Ah, Yes' as at least one person (?First Eng.His initials are A.R) said 'Priority One and Two'!