I would be very hesitant to change routing unless the load level are low.

Isn't it possible to make it onto the first flight standby PHL-ORD but never make the connection on the second flight ORD-SEA if that flight is full or oversold?

You could be stuck in ORD for quite some time, whereas if you keep the original routing, you might just have to wait for the original second flight out of DEN.