I'm not sure about it being a movie but it sounds alot like an episode of Quantum Leap.

http://en.wikipedia.org/wiki/List_of..._Leap_episodes

http://www.dailymotion.com/video/x9f...3-4_shortfilms

Season 3 Episode 1...

Sam leaps into himself as a high school teenager and Al tells him he is there to win a basketball game that was a turning point in many people's lives, but Sam wants to take the opportunity to prevent his sister from marrying an abusive alcoholic, his father from dying from a heart attack, and his brother from getting killed in Vietnam.
Season 3 episode 2...

Sam leaps into Herbert "Magic" Williams (played by Christopher Kirby), a soldier in his brother's platoon in Vietnam, where he is given the chance to keep his brother from dying, but at a terrible price.