I'd try this first...
http://www.microscale.com/Merchant2/merchant.mvc?Screen=PROD&Store_Code=MD&Product_Code=MI-12&Product_Count=&Category_Code=
The Microscale folks make this specifically to salvage old decals. I know there are lots of other 'home remedies' and incantations out there, but this one is made for the job.
Also, my recommendation is to work with an area of the old decal that is of little/no use to the job at hand. In other words, try it on portions of the decal that have no real value to the specific project.
I've only used Decal Film once...and it may have been overkill. I do know that one or two applications according to the instructions will add to the film thickness....make it harder to hide the film later. If that's a deal-breaker, you might want to put the old decals into the hands of an expert custom decal maker to be used as the artwork for making a new fresh set. More expensive, I know, but it really depends on how you value the end result.
Just some thoughts, FWIW....
KD