When is a Courtesy Runner in the game?

[MD Longhorn]

The real answer is ... it's official when the umpire notates it on his lineup card.

Unfortunately, in local leagues slash coed slowpitch ... you often get umpires that don't even keep a lineup card much less notate CR's on it.

In that case, you're kind of up to some made-up notion of when it's official held by the particular umpire on the scene. I would add, however, that just about any umpire I can imagine working would consider the change made when that player took their position on the base. Your guys disagreed, obviously.