I took mine off by carefully using a fingernail. Came right up.. however, I did that the day I bought it. I don't know if it will be any harder for being old. You might try a bit of heat (from a hair dryer- and don't cook the paint) or WD-40 if it is difficult.
As for the paint, it will certainly have oxidized at a different rate than the rest of the bike. If it was garage kept, washed/waxed meticulously it might not be awful; but if it's been in direct sunlight it's entire life and not waxed a lot, you could have a very noticable spot of "fresh" paint vs. the rest of the bike.
If you haven't tried the hair dryer.. that's the best solution to get it to come up. Don't worry too much about cooking the paint.. just keep it in mind, as small as that sticker is you should notice the glue softening up long before you cook the paint.
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