Recent water damage required the removal of our kitchen ceiling below our bathroom. For years we've 'cursed' our toilet for the frequency that it needs to be plunged.

Upon inspection of the plumbing, it was discovered that the drain for the toilet (about 6 feet from toilet to stack), is pitched the wrong way, about a half a bubble on a 4-foot level.

The bathroom plumbing is PVC, tied into the cast iron stack, deep within the walls - access cannot be done DIY.

In order to re-pitch the toilet drain and clean up some other bass-ackwards plumbing connections, I am considering running a new stack, top to bottom, leaving the old cast iron in the walls.

The new stack would come down about 4 feet from the old one, through the walls in a kitchen closet.

What are the issues of leaving the old stack in place, capped on each end?