If someone were presented the option to permanently undo a major aspect of their
Before I could consider the ethics of this scenario, I'd have to satisfy myself that it's a coherent scenario. Let's call the person in question "Jane." The scenario seems to require that something like the following be true: "Jane sacrificed much of her time and energy to achieve justice, but because her sacrificial actions led to an unsatisfying outcome Jane didn't sacrifice much of her time and energy to achieve justice." I can't see how such a scenario is comprehensible enough to be assessed ethically.
The question also arises whether Jane's sacrificial actions contributed so much to Jane's identity -- to who she now is -- that it's incoherent to ask what Jane's life would be like now had she not made those sacrifices: we wouldn't be asking about Jane but about a numerically different person.