The boot stripe on the Mainship 350 and 390's are vinyl with adhesive backing. A heat gun will work, a pinstripe wheel will work, a razor blade (followed by compounding) will work, as will any combination of the above.
putting vinyl adhesive over existing vinyl adhesive will work in many cases, but not something I would risk. Anywhere the old stripe is missing is going to show through. Anywhere where the adhesive let's go on the original stripe, will lift the stripe above it as well. The result will leave a much prouder edge at the top by doubling the thickness and it's tough to get them lined up perfect.
I would personally, completely remove the old stripe, make sure all mold release, wax and adhesive is completely removed, then install the new stripe.