There's a few reasons I changed it...
1) The Empire Strikes Back -
"If he could be turned, he would be a powerful ally."
Why the hell would the Emperor agree to Vader saying, "If I killed you, he would be an acceptable replacement"?
2) The Phantom Menace -
According to George, the rule of two went into effect after the Sith were wiped out, which menas, the only way Yoda could possibly know about it would be if he, himself, were the Sith Master.
I just think it makes more sense if Yoda were speaking about the grander scheme of things, like "There is another..."
It's just common sense...
The last Sith wouldn't have waited a thousand years to stick his ass out on the line in an uncertain situation and get killed by a padawan... There has to be more than one.
Plus it adds the possibility of Vader, Palpatine, and Luke co-existing as Sith in the OT... a sort of Evil trinity.
I just think it works better.