If you're trying to take a productive hive and make several productive hives for next year, you want to replicate those hive genes. Introducing outside queens isn't going to guarantee productive hives, although grafting cells from that hive isn't a guarantee of productive hives either, since the next generation of queens will lay workers with mixed genes from other drones.
"Too late" is completely up to your discretion. Will there be another flow to draw comb or will you be feeding all Fall?