It is absolutely possible to do what the OP is asking. The change would have to be made in NRT (or at least once the OP is in NRT; last time I did it over the phone back to the US res center), after the ANA segment is flown, but it is absolutely possible.
The "no routing change on standby" doesn't matter because the OP will be confirming SDC, not flying standby.
If booked LAS-IAH-EWR you would absolutely be able to confirm on LAS-EWR, assuming the inventory is available.