Andes diverted the Amazon, how?

[BigMoosie]

It is generally accepted that the Amazon basin once drained into the Pacific, and that when the Andes formed it begin flowing in the opposite direction. I am wondering what the process would have been like, I see alternatives:

 

* Would it have changed direction immediately once a certain threshold was met?

* Would there have been two separate basins flowing in separate directions and the Eastern one slowly grew in size?

* bWould a large part of South America be flooded into a giant dam, with outlets on both sides?

 

I cannot clearly see what would have happened.