The issue isn't in thrust or fuel tolerances, the rockets used on the core stage are only able to be fired once since they use pyrotechnic charges to get the turbines to start spinning on the pad. Replacing them would mean completely redesigning the entire rocket, you can't just take off one rocket engine and add a different one like in Kerbal Space Program, it takes years of rigorous testing and certifying. And it would require many more launches of core stages we have no guarantee would come down from orbit anyway.