Based on doing that change but off the Overground be towards the front of the train in trains out of central London/rear of the train for trains into London and use the nearest stairs to you. It is indeed via the subway.When alighting GA trains at Stratford and heading to the Jubilee line I seem to always end up having to use the escalators up and down across the DLR to get there. Presumably the trick is to continue walking along the platform and go down one of the later steps down to the subway?