Old decals

I just attempted a removal on a  model and it didn't go well.   My intent was not to reuse but when I put the micro sol on and then the vinegar they would not budge as if they were painted on.  Then I used lacquer thinner.   It removed all the paint even underneath the decal and the decal ended up intact and fixed to the bare plastic.   It was at that point I resorted to an exacto knife to do the deed.  

I don't know what you will encounter.

Just what  are these decals and what makes them so important?

Scan the decals and reprint on your own decal paper.

Theyre very old school slot car racing decals on HO scale cars. They just  have a lot of appeal for car collectors.

Are these decals already fixed to a model?

If so, then, they can't be reused, as it is improbable that they can be removed without disintegrating.

You may get better milage in photographing/scanning them & reprinting on decal paper?

If these are old decals on a backing sheet, then as above, scan, edit/clean up & reprint on decal paper.

There are products that you paint over old decals that can effectively varnish them enough to hold together for application, such as Microscale Film, but often won't have enough adhesive left to stick to the model....

Pretty good Info/explanation here from  Old Model Kits

Good Luck