When do Black Sisters take their other oaths?

[The Lost One]

I have been reading the boards quite a bit and it got me thinking about the Black Ajah and their dark oaths.  When do they take them?

 

I am assuming if a darkfriend accepted is raised, she still takes the normal 3 oaths then rushes to one of her other meetings, has the 3 oaths removed and takes new ones.  Or if an Aes Sedai becomes black later, they remove the 3 oaths at that time and she takes the dark oaths (or whatever we are calling them).

 

Why is this important?  I have seen many people defend sisters from being black because they took the 3 oaths.  But we could have seen each and every sister take the 3 oaths in the books, only to be missing when they ran to one of their other meetings and had them removed. 

 

Am my missing something?  Unless we specifically know a sister is not Black, even if they have taken the 3 oaths, they still could be Black Ajah, right?

[Ealdur Tinuviel]

Right. In fact, the very same oath rod used to administer the 3 Oaths is used to take it away.

 

But the Tower only has one oath rod, so the BA has to make sure no one is using it when they have their ceremonies.