For removing lettering without damaging the paint you can also try a paste of Abrasive Cleanser like Comet or Ajax, take a bit of powder and make a soft paste out of it with a drop or two of water. Then use a toothpick or piece of stripwood to rub the lettering on the car with the paste. It takes off the lettering gently and quickly and won't really harm the paint underneath if you are careful. It will burnish the paint making it somewhat glossy, but since you'll have to coat it with gloss to re-decal it anyway this is not a problem. I learned this from a friend and it works very well, and gives you a LOT of control over the process which brake fluid or strippers sometimes don't.
BTW a tip - rub vertically on the side, not horizontally or in a circular motion - if there are any 'streaks' left over after cleaning off the paste, they will look like weathering and not like you rubbed something off with an eraser.