Ali Duran Leonard Robinson All four had fights where they needed to change tactics in the heat of combat against prime competition to get the win.
Ali completely changed his gameplan in the Rumble in the Jungle after the 1st round. One of the most brilliant tactical maneuvers in the history of sports.
Well Ali and Leonard certainly switched tactics and implemented a Plan B to great effect when the stakes couldn't have been any higher and the opponents couldn't have been more dangerous (against Foreman and Hearns 1). I don't know how often boxers have managed to totally change their styles during a fight to overcome obstacles and win. Maybe that's why only a few spring to mind and stand out so much? I think most Plan B's are implmented in a rematch. Fighters tend to stick to the same strategy during a fight because their trainers want them to stick to the plan they worked on in camp (whether it's working or not). Interestingly, Angelo Dundee was the trainer for both the Ali and Leonard examples. He told Ali "not" to lay on the ropes against Foreman, but Ali ignored him. In 1981, Dundee famously told Leonard he was blowing it and he had to go for it against Hearns.
Robison did it against Turpin 2nd fight. Duran did it against Dejesus 2nd fight, which lead to Dejesus getting dominated the 3rd fight. also against Barkley.
I always thought Lennox Lewis switched well between boxing smart or blasting a guy out quickly if he could
Hopkins was one of my first thoughts too, but I think it was more he just got his plan A right rather than being particularly adept at switching on the fly. The same really applies to Ward.
Kovalev fits the bill, but Hopkins started walking Tito down because he'd beat him so bad he started backing off. It was a dominance thing rather than a plan B
Some fighters are problem-solvers. They can adjust in-fight and solve or at least more effectively deal with whatever’s happening that wasn’t working (or was working for the opponent). Someone mentioned Lennox Lewis sometimes getting the quick kill and other times deciding to box. I think that’s a product of Emanuel Steward in the corner to a large degree. Manny would often have a multi-part game plan going into a fight. He said the game plan for Thomas Hearns vs. Pipino Cuevas was: Go after him, be the aggressor, throw heavy artillery and try to get him out of there the first three rounds. (Obviously this part of the plan worked and there was no need to shift after, but had there been …) Then box Cuevas for the next three rounds. And then go after him again for rounds 7-9. He explained this in postfight interviews. A reporter asked him what was the plan after that. Steward laughed and said there was no plan after that because no way in hell that it wouldn’t have ended in a KO by that time one way or the other, haha.
Marvin Hagler cleverly switched southpaw after giving away the first four rounds (and thus the fight) to Sugar Ray Leonard. If it had been scheduled for 20, at some point he would have shifted game plan again and tried to cut off the ring.