It may require quite a bit of reverse engineering, it helps if you have the pin out of both the old and new DRO.
Even then, many modern DRO's the scale is graduated in metric divisions, the conversion to imperial is done in the DRO unit.
Also whether the new unit can be scaled to suit your old scale resolution.
You could hook them up to a PC like this:
http://www.lindsayengraving.com/othe...rests/dro.html
Al.