Replacing some line to install point of use water heater.. Have one old elbow that will not break loose. Is there some magic solution to help break the old seal?

You didn't say but I'm assuming the elbo is threaded galvanized. Tap around the threads to loosen them up and load the threads up with WD 40 and let it set for awhile. Then put a 18" pipe wrench on that sucker and "gorilla" it out.
Good luck, Tom

There might be some helpful hints for you in this old thread:
https://www.askmehelpdesk.com/plumbing/cast-iron-plug-stuck-3-x-1-5-tee-101860.html

I had a real bear of a time getting the plug out, and was very afraid to break the fitting. If you have the same rector-seal type goop on there, I highly recommend an adhesive solvent like Goof-Off or Goo-Gone.

~aaron

Adding a couple of points:

Apply the WD-40 before the hammer. Hammering opens up the joint allowing the liquid to wick in.

Have somebody else hold the pipe with a 24'' wrench when you apply the 18'' to the elbow. I sometimes delegate holding things to my Vice-Grips.