Hardly 'highly confidential', the possibility was in the news immediately.
I was hitchhiking through Eastern Europe that summer and got to Greece.
I heard it in a VOA broadcast one night in late August as I dined in
a small trattoria on the bay on Mytilene Island in the Aegean, with
the moon reflecting in the bay waters. It was a magical moment.

"Henry Spencer" wrote
It wasn't "last minute" -- preparation for it was underway in mid-August,
but the change was tentative and highly confidential until after Apollo 7
flew successfully in October. But yes, fears of a manned Zond figured
into that decision.