I once heard an acquaintance of a friend of mine put it this way (back when I wasn't familiar with transhumanism and I didn't know whether he was wrong or right): “Life extension had better be called old age extension: you may die at 160 instead of 80, but it's not like when you're 40 you'll do the things people today do when they're 20.”
Anyway, going by revealed preferences, fewer people might dislike the former scenario than you might think.
I don't think that necessarily reveals people's preferences; that would imply that they choose that outcome. I think in most cases people are ignorant of what is going to happen, or know only in an abstract sense. Those who actually know what they're in for, tend not to die that way..
This new study by Pew Research on American opinions about radical life extension turned up some interesting results:
I also find the demographic splits on page 3 to be surprising. On the question of whether treatments to extend life by decades would be a good thing for society, whites are significantly less likely to agree: 36% of whites agree whereas 48% of Hispanics and 56% of blacks do. There is a negative correlation with age (48% of adults 18-29, 46% of adults 30-49, 37% of adults 50-64, 31% of adults 65 and older) and with income (47% of those earning 30k and less, 42% of those earning from 30k-75k, and 39% of those earning 75k+). The income result in particular surprises me, as my intuition was that people with a higher quality of life would be significantly more pro-life extension.