Be able to take ability back points placed before 20 minutes

I am sure everyone has done it once before. Placed a point into a spell at base. I have done it many times, and that is why I am proposing something new for Riot to add. If you point in a spell before 20 seconds into the game, you should be allowed to undo that, and be able to point into another spell. I can see no cons to this idea, as before 20 seconds you do not come into contact with the enemy team, minions, turrets, or monsters; changing it would make no impact on the game, other than undoing a mistake.