Most changes can undone, but the effort can be clusmy. Especially if it's stuff that could define the character. A three year run in which Peter Parker kills, becomes a father, cheats on the mother of his child and celebrates his 35th birthday can be reversed, but it's not going to be pretty. That's especially true if changes are more successful, with an impact on the Marvel Universe.
Those are pretty extreme examples, but if they were done well, anything is possible. If they were done poorly, then the easier they're erased. Spider-Man once punched his pregnant wife, he'll recover from just about anything. Hal Jordan slaughtered the whole Corps and tried to destroy the sun, he recovered.