Pluto didn't really get demoted. It was reclassified, and that decision is still being debated by scientists today.
Often, the first step in solving scientific problems is to classify the groups of objects being studied. The difference between stars and planets is pretty obvious: stars make their own light and planets don't - planets only reflect light. Planets orbit stars. Moons orbit planets. So far, so good.
What's an asteroid? When Ceres was discovered on January 1, 1801, it was considered the 8th planet, the one that seemed to be missing between Mars and Jupiter. At the time, the other planets known were Mercury, Venus, Earth, Saturn, and Uranus. Within 7 years, three more "planets" were found between Mars and Jupiter! By 1850 or so, the list was getting out of hand: now a total of 13 "planets" were known between Mars and Jupiter.
During this decade astronomers realized these objects were different, being much smaller than the planets previously known and these new ones were all in the space between Mars and Jupiter. Ceres and the others were reclassified to being "minor planets" or asteroids, and we now have cataloged well over 600,000 of them, with more added to the list each week.
When Pluto was discovered in 1930, it was thought to be a predicted Planet X. While it was somewhat close to the predicted position, it was nowhere near large enough to fit the prediction. Still, it was there, apparently by itself, and so it was called a planet. Flash forward to 1992, and smaller objects with distant orbits like Pluto's started being discovered.
More than 2,000 are now known.
One of them, Eris, discovered in 2006, seemed to be larger than Pluto. Is it a planet, like Pluto? Or should Pluto, Eris, and the many smaller ones be reclassified to reflect their differences from the planets Mercury-Neptune because they are all smaller and more distant and part of a large collection? (Just as the asteroids are smaller, limited in space, and part of a large collection.) The International Astronomical Union (IAU) decided they should be re-classified as a group, called Trans-Neptunian Objects, TNOs, since their orbits are larger than Neptune's. There were also other reasons to change the classification.
The IAU also decided to recognize one particular characteristic of the largest of the asteroids and TNOs. The largest are spherical, like a soccer ball, but the smaller ones are all more or less potato or peanut shaped. The IAU said that spherical asteroids and TNOs should be classified as dwarf planets. Five are now recognized, including Pluto, Eris (new data says it's slightly smaller than Pluto), Makemake, Haumea and Ceres.
So this is why and how Pluto was reclassified. In the process, it went from being the ninth planet to being the No. 1 Trans_Neptunian Object. Either way Pluto and its companions and its asteroid cousins are important to planetary
scientists. In the process astronomers developed better ideas for classifying objects in the solar system and elsewhere, so science has made progress.
There were several proposals for missions to Pluto. The earliest I know of were in the plans for the Grand Tour, developed in the 1960s using the TOPS spacecraft design. The Grand Tour would have permitted a gravity assist trajectory from Jupiter-Saturn-Uranus-Neptune-Pluto.
The history next is a little muddy. Four large outer planet spacecraft were planned, under the name Viking, that were to be launched on two Saturn V rockets. Pluto was included in the mission plans, with Jupiter, Saturn, Uranus, and Neptune. I believe the proposed trajectories were Jupiter-Saturn-Pluto and Jupiter-Uranus-Neptune. (In that same time period, large Mars landers called Voyagers were going to be launched on Saturn Vs too. They were down-sized and renamed Viking 1 and 2 and launched on smaller boosters. Apparently names ended up being swapped, as you'll read in the next paragraph.)
The Viking effort to the outer planets evolved into the smaller Voyager 1 and Voyager 2 spacecraft which were launched individually on smaller Titan IIIC rockets. They had trajectories called JST (Jupiter-Saturn-Titan) for Voyager 1 and JSX (Jupiter-Saturn-To be Determined) for Voyager 2, assuming they lasted longer than their primary missions to Jupiter and Saturn. VGR2 was a back-up to VGR1 and would have flown its own JST if VGR1 had not completed its mission or if Titan had been more interesting than was observed at the time (Cassini has shown us that it is MUCH MORE interesting than anyone dreamed). Titan's hazy atmosphere limited what could be done so VGR2 could have gone to Pluto or to Uranus and Neptune. The choice went to Uranus and Neptune, though an acquaintance of mine did run the trajectory long after the decision and found that VGR2 would have had to burrow through Neptune to reach Pluto.
For a more detailed account of more recent efforts leading up to New Horizons, I refer you to http://www.planetary.org/explore/topics/new_horizons/road_to_nh.html. It will recount a little of what I've described and much more about Pluto missions that were proposed but didn't get far along in development.
I cannot cite specific examples of New Horizons technology being used by subsequent missions. What I can tell you, though, is that engineers will always use "heritage" technology if it will do the necessary job. Using heritage technology saves a lot of money in development and testing, compared to brand new technology. You can be sure that New Horizons will provide heritage for instruments or engineering subsystems for future missions.