I don't quite see the relevance of a shortage of 313s now, to something that is not required for 18 months, to be honest. The shift of more 377s onto Metro services is dependent on platform lengthening in South London as well - that's not due for completion until 2013, and far more likely to be delayed as it's down to Network Rail...
Apparently the shift of 377s is not dependent on platform lengthening on all metro routes and then there is the issue of the 456s.