Obviously, this is a fairly unique situation, but Robert Stock is one example of a player who was technically considered a recruited walk-on his freshman year but made a big contribution to his team.
Quoted from Baseball America article:
"The Stocks were the ones who first broached the possibility of early enrollment with the USC coaching staff, and they were told the Trojans had no more scholarship money to give out. But the Stocks decided getting to school early was important enough that they would pay Robert's way his first year."