As you phrased it, life extension and replacement seem roughly similar to me. I don't feel the need to modify my utilitarianism to strongly prefer life extension. There are some differences, though:

• Perhaps the later life years are less pleasant than the earlier ones? You're less physically able, more cynical, less open to new ideas? Or perhaps the later life years are more pleasant than the earlier ones? You've had the time to get deeply into subjects and achieve mastery, you could have some very strong old friendships, you have a better model of the world.
• People who live longer might be better at solving the worlds problems. Or having more different people might be better.
• I think of the harm of death as being the removal of the potential for future joy combined with the suffering of those who remain, and replacement deals with the former. While death seems to cause pain to those who continue living in all societies, death of "old age" doesn't seem to cause too much suffering to other people.

I would definitely (selfishly) prefer life extension.

(I'm a total utilitarian)