Personally I use a flat blade exacto knife to raise the insert decals I am removing. Most of the time they just pop off with very little scraping.
I usually wet sand the inserts with 800-1000 paper with the tip of my finger to get out the rough scratches. Then I use a syringe to put a little clear in the void to bring it up level with the with playfield. Lay a couple coats of clear on the playfield to blend them all together. Block wet sand the playfield again w/800-1000 paper, unless you have a DA, then I use the hard surface pad. Prep the playfield for clear then install the decals. I usually spray some Bull Dog adhesion promoter on the decals before I clear to help the clear adhere better to the insert decals. Then I spray a few more coats of clear, (the first being a dusting), wet sand again. And you will probably need to apply a few more coats of clear to bring it up level with the insert decals..
I never install decals on a playfield that has had the insert decals removed without clear coating first. The edges of the removed inserts are not smooth and you will almost always have some air under the edges. Once you get air under them and seal it on with clear your screwed. They will always have a little fogging along the edge and they will raise much easier.