Hull stripes that are not totally degraded by UV can be removed fairly easily by careful use of a hairdryer or hot air gun (be very careful if you use this, only gentle heat is required). The idea is just to soften the adhesive enough to be able to peel it back. We have the same job to do on coachroof stripes that have had 6 years of Med sun and have crazed, word is these will be very difficult to remove. We have obtained some special solvent from SAC Graphics (01384 441787) but they warned it would still be a slow process, haven't had the weather yet to try it out.
Done lots of this working for westerley Sea School in late 80's, dont use hair dryer, it will leave adhesive on the hull, just use good sharp blade very carefully to lift one end then gently pull slowly, slower the better, should break less often and not leave glue behind.
Then I used number 3 thinnners to remove any residue.
I used to remove loads of stripes during fitting out when I was a flotilla skipper in the med. (well, those that our customers had not left on the quayside or other boats).
A sharp blade, plenty of care, and acetone to remove the glue seemed to be the best approach.