How to upgrade old sewer drain?
Before resorting to demo you might hire a plumber to use a drain camera to scope upstream and downstream of the vertical cleanout, it might cost $60 - $100 but would be money well spent if it saves you some jack hammering and digging.

There are some pretty effective and long lasting methods of rehabilitating old sewer pipe by "shooting" a new internal pipe lining, the liners will even bridge cracks and small breaks. It involves high-pressure hydroblasting to clean the I.D., and opening up the line for access, but maybe that could be done from the exposed portion above the floor and 16" deep well. It may be a good alternative to extensive concrete demo and excavation.