Directly answering the question, organ replacement including some brain augmentation that shifts into uploading eventually seems most likely. Cryonics isn't really a direct answer to the question, if you want to talk about trajectories to achieve immortality.... It's too hard to predict where a breakthrough will be made or a wall will be hit. I think the most feasible trajectory is focusing on money and power, then organ replacement and traditional life extension techniques, which include general existential risk reduction.