"The Immortal Superman"
When Superman receives a distress call from the future, the only way he can answer it is via a Legion Time Bubble. Once he has reached the future via the bubble, he is horrified to find that his body has aged 100,000 years.
This story runs from issue #385 to #387.
"Tales of the Legion of Super-Heroes: The Fallen Star Boy"
Star Boy wants in on the mission when it concerns his home planet. Once he and the other Legionaries reach Xanthu, he finds that it all a set-up to get revenge on him.