It's held on with 3M double sided tape. You would need to saw through the sticky tape WITHOUT bending, or pulling outward, on the trim. Keep it straight as possible. The danger zone is the door strips. If you're careful, you can separate them from the body, and use some masking tape to hold it up against the body on the part you've already done. Just keep working down the side slowly and tape it up as you go so it doesn't bend and crack that mylar chrome. Cleaning off the old tape from the back would be a huge PITA, but new ones are prehistoric and probably getting transformed into crude oil.
It'll take some luck to get them off without any damage. Since yours is an 87, yours will be black. The body side molding was the same color as the upper body for 86 and 87. In 1985, all 442 cars had body side molding with lower body color, which the closest color was gunmetal gray molding, regardless of upper body color. I have done a test that SEM black vinyl dye holds up in the weather just fine over gunmetal gray.
There are some "almost" correct body side moldings out there with a bit of texture and a hair narrower at 5/8" (factory stuff is 3/4"). But unless you're ok with that, and absolutely have to have the GM stuff, that junk evaporated years ago. You will need 16 feet total. Plus a super-sharp trimming knife (larger x-acto or similar) and the ability to cut and make birdmouths to emulate the original. If you have to go this route, keep all your old molding you pull off to use as templates. That's the best advice I can give you.
Here's an ad for two 10 foot pieces of the black/chrome 5/8" molding. Again, it's slightly different than the original but it's all what you want and can live with.
Find many great new & used options and get the best deals for Vintage Style 5/8" Black & Chrome Side Body Trim Molding at the best online prices at eBay! Free shipping for many products!
I made a "chart" for myself for installing new body side molding. The most interesting part is that the molding only has finished ends on the most forward piece and the most rearward piece, and on either side of the rear wheelwell. Otherwise it's just cut in place. If you have door edge moldings, you need to trim 1/8" off the length of the door molding. If not, you don't. There's also an angle cut on the front of the door pieces. Should look like this, looking down on top of the driver door front.
I'm ready with some gunmetal ones for my 85. Or, I may end up having to sell this junk one day.