In Jack's blogathon, you simply recast performances from 2013 that you hated/liked. Here are a few I'd change.
Rebecca Hall for Rachel McAdams in About Time
McAdams just didn't feel right in this film, but Hall would've been a nice addition (and stronger acting choice) to the Richard Curtis' world.
Garrett Hedlund for Charlie Hunnam in Pacific Rim
While Hunnam did the best he could, Hedlund is the more proven actor, which he demonstrated in On the Road.
Michael Shannon for Guy Pearce in Iron Man 3
Pearce was fine. Shannon could've brought more intensity to the villain, though.
Jim Caviezel for Tom Cruise in Oblivion
Cruise was arguably miscast, and Caviezel could've given the character more emotional depth.
Kristin Scott Thomas for Catherine Zeta-Jones in Side Effects
Too on-the-nose, perhaps, but she'd bring a fierceness to the role that was lacking.