To remove striping: Use an eraser wheel. It is a rubber type wheel approx 4" in diameter that you mount into an electric drill and literally erase the decal. Surprisingly it does not harm the surface under the decal. It is often used on boats, cars & trucks.
New stripes should not be put over old stripes as they will either not stick good or distort over time.
All stripes and stripping materials can be found at a good auto body & paint supply shop. Or look at http://www.eastwoodco.com/. The erasers are shown on http://eastwood.resultspage.com/search?p=Q&ts=custom&w=eraser
Enjoy,
Robert Flight
Rochester, NY