I have a 1986 Oles with a V8 engine and 125,000 plus miles and the spark plugs have never been replace. My question is should I be concerned about them breaking off leaving the threads in the head? If so what can I do to prevent it?

Yes. Only try removing the plugs when the engine is "stone cold," to lessen the chance of stripping the threads in the aluminum head. Take it real easy and be sensitive to the feel of how it's progressing. Before you begin the job, use Kroil or PB Blaster and allow it to sit overnight. Plugs should really be removed and replaced every several years, to prevent seizure. This is particularly important on platinum plugs, even if you decide to reinstall them. My experience is that platinum plugs should be replaced at about half the mileage recommended in the owner's manual. If they all come out safely, reinstall with half a pea-size drop of anti-seize compound on the lower threads. Allow rotation to spread the compound, as they are tightened. Reduce spark plug torque 25%, since anti-seize is a lubricant, to prevent stripping threads.

I agree with txgreasemonkey although I'm guessing the heads are iron, regardless,
Use of pb blaster ahead of time will definitely give you an advantage.