That was before TMP, and even then he had concerns about the actors being too old for a prequel!
GR was always open to recasting. He assumed TMP would be attracting acting teams such as Paul Newman and Robert Redford as Kirk and Spock (or was that Spock and Kirk?). The first Walter Koenig knew of ST:TMP in pre-production was when GR invited him to consider playing Chekov's father in the movie. (See "Chekov's Enterprise" by Koenig.)
Exactly. In the Making of TMP book, it's even stated that GR knew a day would come when the parts would be recast. The only reason he objected with Star Trek: The First Adventure (the Bennett Academy movie) is because he thought he should have a hand in the film and its recast. That and he probably felt that he had better clot with TNG up and running.