I don't think anyone's been offerred the chance just to pop off to another stream. It was mentioned that anyone not shining might get "asked to volunteer" to go to another stream. The impression though is that as long as you're passing, you'll still get to go to Valley.
I hear on the grapevine that things will be slightly different for future courses at BFJT though; more along the lines of chopping early if people aren't getting better than average, and people who fail to pass Prog Cx not getting offerred restreaming.