Putting new decals over old.

I’d also remove the old ones first,,but if you’re putting matching decals on, they will last just as long on top of your current decals as they would otherwise and presumably look right.

Yes you can use a heat gun and scraper to remove the decals. The trick is to get ALL the adhesive off also.

I agree with Gdetrailer on this. Do it right the first time or you will be doing it again.

You can supposedly remove the decals with a heat gun and putty knife. I'd try that first.

If you want the new decals to fail even faster than slap them right over the existing decals.

Just like anything else in life, a key to everything is having a "good foundation", old decals are worn out, unstable which ultimately will result in damaging anything slapped on top of them.

Good prep by removing old will reward you with the longest life of your new decals.

There are specialized decal adhesive remover tools and chemicals that will make the removal and cleanup job go smoother..

If you don't want to put the time and effort into doing the job right, take it to a body shop and pay the price to get the job done correctly.