No, you, or who you heard this from, are getting things mixed up. It was never actually intended that Obi-Wan killed Luke's father, it was that in the original script, Darth Vader told Luke that Obi-Wan killed his father. Right before the scene was filmed, they told Mark Hamil that regardless of what the script says, or what David Prowse is saying, what you are hearing is "Luke, I am your father." Then later when James Earl Jones recorded Darth's lines, they had him say the actual words that are heard in the film.

The only reason they did this was to avoid any spoilers leaking since so many of the scripts would be floating around. That is why they didn't even tell Mark until right before he filmed it. That way if anything leak, it would be misinformation.

Also I am pretty sure Lucas was the one to come up with the "I am your father idea", not Kershner.