I can´t find any reference in the Automation Help to instruct me how to programatically change the sketch support of a positioned sketch, from one face of a pad to the opposite.
You will probably need to set a new absolute axis for the sketch. This can be obtained by using the selected element get Coordinates and click twice in different areas on the face and create the plane matrix from that. If you did not do it interactively then you could calculate the new coordinates if it was a pad from Limit2 depth and a vector direction.