If utility per life year is equal, and total life years are equal, then total utility is equal and total utilitarianism is indifferent. But for the question to be relevant for decision-making purposes, you have to keep constant not utility itself, but various inputs to utility, such as wealth. Nobody is facing the problem of how to distribute a fixed utility budget. (And then after that, of course, you can analyze how those inputs themselves would vary as a result of life extension.)

I object to the phrasing "utility experienced". Utility isn't something you experience, it's a statement about a regularity in someone's preference ordering -- in this case, mine.