flooper, I recommend that you have the Cast Iron portion of your sewer pipeline inspected with the use of a sewer line camera. If the condition of the cast iron pipe is poor, then have it replaced. If it is in good condition, then “don’t fix it if it ain’t broke.”
If the Cast Iron pipe is in poor condition, and is allowing infiltration to occur, then investigate the technology of “trenchless” pipe replacement. You can learn about this technology from a website provided by Dale’s Plumbing Service of Hutchinson, Kansas. His website address is:
http://www.dalestrenchless.com
Typically, Cast Iron pipe has a very long service life, 100 years or more. This pipeline is only 43 years old, so if it was installed correctly, I suspect that it will provide many more years of good service.