When 150 guns redefine the ship-rate system

Would Victory become a second or third rate if 150-plus-gun first rates became standard?

Yes; it would probably become a third rate under a larger revised scale.

A fleet in which 150-gun ships became normal first rates would need new rating boundaries. Second rates might cover roughly 120 to 150 guns, with third rates around 90 to 120, depending on the period. Victory's historic first-rate status would not remain fixed under that larger scale. The labels would shift with the fleet's new upper end and move her into a lower rate.