If the decal is glued to "paper" side of the paper (instead on the shiny one), then the glue is now useless.
It's pretty old - there is a good chance that the glue itself is old, regardless of where it is applied.
Long story short, buy another decal, which is not burdened by those problems.
We don't recommend products here, but my hack is:
You have a few days to experiment with different types of bandage and oil, on some other part of your body, to find out which combination is most easily removed.
The question suggests you might be thinking of using a plaster like in the left picture:
Thinking outside the box, you can keep adhesive ...
The screws help to hold the lens adjustments from being changed. You would want them loose to change your adjustment and then to tighten the screws to hold the changes that you made for different lighting or subject positions.
Instead of glue to hold the screw positions, why not use a wide elastic band (shown in pink) around the lens (in black) covering the ...
Baby oil is very much a uniform product across brands -- light mineral oil with scent added. You could use any commercial brand, or mineral oil sold as a laxative, or light machine oil as is used for guns or sewing machines, with effectively the same results except they smell different.
Adhesive bandages come in broadly three classes: cheap, common, and ...
I have taken off many, many decals, stickers and signage at work with a hairdryer. Never ever seen one that could de reglued somewhere else. They seem to always get damaged.
Ps. Do not try to take them off on cold days, if you truly want to try and salvage them. Again I have never seen them undamaged when being peeled off.
My best suggestion is that you ...
I recently discovered some 3M products for this. I was bonding silicone to plastic, but I'm sure it will repair a tear. You'll need two products: 3M AC79 primer and PR100 adhesive. Apply the AC79 first, and then apply the PR100 on top of that. I think that should work well for you if you haven't already resolved this.