Richard wrote:

Long ago, I was told by a BBC producer that David Attenborough's original classic series, Life on Earth, was revoiced for the American market by Robert Redford, but I can find no confirmation of this. Does anyone know whether it is true, or definitely false?

A quick search found no confirmation. A letter signed by Michael Peacock of Warner Bros TV in London, printed in New Scientist on 6 August 1981, quite indignantly denies a number of inaccuracies that had apparently earlier been reported about Life on Earth, first among them the following:

There never was a Warners plan to "cut out much of stuffy David Attenborough" and to replace him with Robert Redford.

Unless someone can up with something at least as authoritative pointing in the other direction, that probably answers the question.