Changing a number without reporting the change ???
 

Penalized if discovered while being violated: A player shall not participate after changing a number without reporting the change to the scorer and an official; Player flagrant technical foul.

What if said player participates, unbeknown to an official, and is the substituted for, and is sitting on the bench? Is it too late to penalize? Can it be penalized the next time the player enters the game?

Yes, and yes.