I didn't like the R2D2 'rockets'. According to Entertainment Weekly, those R2D2 rockets have a 20 year warranty! That's why we never see them again.
C3PO looks very different 20 years later in ep4. Also, droids are like appliances in the Star Wars universe. Would you recognize your Maytag refrigerator 20 years later after a complete overhaul? Also, the droids have programmed intelligence. They are just machines, after all. Their long-term memory of people, events, and places seems lacking. That's one way to rationalize it.
~ Bret ~