1. EGR problems could cause future O2 sensor (or other) fault codes. Your 0401 code (Insufficient EGR Flow) seems to indicate clogged up EGR passages, a vacuum leak to EGR valve, or some other problem.
2. Yes, all the sensorcs can be changed from either the top of the engine, (Upper sensors) or by jacking up the car so you can slide underneath to get a wrench on lower sensors on the exhaust/y-pipe. (I think the 0420 code would indicate problems with the pre-cat or the lower sensor rather than the upper sensor)
I would try to clear up the EGR problem before swapping out O2 sensors. A good EGR cleaning can do wonders. Also check for cracked or disconnected hoses.
