Placekick holders knee

Question
I'm aware of the knee down rule exception for the holder on place kicks, however, I'm not sure at what point the holder changes to a ball carrier. Specifically, may the holder come off his knee to field an errant snap and then return to his knee to complete the kick? Or, does he become a ball carrier once he fields the ball off his knee?

Answer
DID NOT REALIZE MY RESPONSE WAS NOT CLEAR, SORRY ABOUT THAT. LET ME TRY TO CLEAR IT UP...
YOUR QUESTION "may the holder come off his knee to field an errant snap and then return to his knee to complete the kick?" - ANSWER: YES.

YOUR QUESTION "what point the holder changes to a ball carrier?"
ANSWER - WHEN HE RUNS WITH THE BALL AND DOES NOT INTEND TO RETURN TO THE GROUND TO HOLD FOR A KICK, WHEN HE IS ON HIS FEET CONTROLING THE BALL AND MOVING AWAY FROM THE KICKER; NOT MAKING AN ATTEMPT TO RETURN TO THE GROUND TO HOLD OR WHEN THERE IS NO KICKER IN POSITION TO MAKE THE KICKLACE TO MAKE .  

I HOPE THIS EXPLAINS THE RULE.

The Holder may rise to catch the errant snap and return to the ground to hold the ball and the ball remains live.  

Rule 4-1-3-b addresses this situation.  The ball becomes dead  by rule:
When any part of the ball carrier抯 body, except his hand or foot, touches the ground or when the ball carrier is tackled or otherwise falls and loses possession of the ball as he contacts the ground with any part of his body, except his hand or foot [Exception: The ball remains alive when an offensive player has simulated a kick or is in position to kick the ball held for a place kick by a teammate. The ball may be kicked, passed or advanced by rule]

A holder is defined as a player who controls the ball on the ground or on a kicking tee. During a scrimmage-kick play, he remains the holder until no player is in position to make the kick or, if the ball is kicked, until the kicker has had a reasonable time to regain his balance.

The runner is a player in possession of a live ball or simulating possession of a live ball. A ball carrier is a runner in possession of a live ball.

Vic Winnek
NCAA Football Official