If a properly installed steel cable can be placed well above the strangling cable, and is tensioned enough to lightly slacken the old one?
I see no reason the older cable can't be removed once the new cable is in.
Do not try and remove the actual encircling old cable on each trunk, simply sever it twice with a very sharp chisel, at points 180 degrees apart on the 360 encirclement, and leave it in place. Don't try to dig out the encircling cable in the trunk.
If done properly, the prognosis for the old oak is very good in my opinion.
Nice tree, well worth saving, no doubt.
Sub it to a cabling expert who knows what he's doing, uses steel cables and throughbolts, and doesn't use that synthetic cobra crap.
Good eye Single Jack!
jomoco