Using the last year in college and current NBA season:
TOP 10 REBOUNDING SMALL FORWARDS (who played in college)
Carmelo Anthony: 10 rpg in college, 7.7 rpg in the NBA (-2.3)
Kevin Durant: 11.1 rpg in college, 7.2 rpg in the NBA (-3.9)
Shawn Marion: 9.3 rpg in college, 6.6 rpg in the NBA (-2.7)
Rudy Gay: 6.4 rpg in college, 6.2 rpg in the NBA (-0.2)
Luol Deng: 6.9 rpg in college, 6.1 rpg in the NBA (-0.8)
Wilson Chandler: 6.9 rpg in college, 5.9 rpg in the NBA (-1)
Danny Granger: 8.9 rpg in college, 5.6 rpg in the NBA (-3.3)
Trevor Ariza: 6.5 rpg in college, 5.5 rpg in the NBA (-1)
Luc Richard Mbah a Moute: 6 rpg in college, 5.3 rpg in the NBA (-0.7)
Paul Pierce: 6.3 rpg in college, 6.1 rpg in the NBA (-0.2) (Had to use career averages)
This isn't sure fire proof, since its only one season and not career averages (except for Pierce), but it would appear that Small Forwards typically rebound less upon entering the NBA. Which is something you would expect. So, in one sense, CBB Fan25 is right in saying that rebounding will decrease, but the 2-3 number isn't correct. It would appear that that number is typically around 1-1.5 less rebounds.
Of course, this ignores so many factors (what was the player's role in college? What was their role in the NBA? What did they average in their careers? Who were they playing with?) that its probably not even a safe analysis.