Were these probes designed with instruments to be used specifically when they got past, say the orbit of Pluto? Or have we just been using the normal NASA kung-fu and making use of instruments originally designed for other purposes?

Or to put it another way, was the extended data collection mission for these probes an after-thought, as in "Hey, as long as we're in the neighborhood...", or did they know when they were launched they'd be collecting data from them 30 years later?

Rob