How do you determine whether you are still yourself after a given period of time?
Well, how do you do it now?
For my own part, I don't think the question means anything. I will change over time; I have already changed over time. As long as the transitions are relatively gradual, there won't be any complaints on that score.
Terminal values and preferences are not rational or irrational. They simply are your preferences. I want a pizza. If I get a pizza, that won't make me consent to get shot. I still want a pizza. There are a virtually infinite number of me that DO have a pizza. I still want a pizza. The pizza from a certain point of view won't exist, and neither will I, by the time I get to eat some of it. I still want a pizza, damn it.
Of course, if you think all of that is irrational, then by all means don't order the pizza. More for me."