Bought a 2 bedroom 1 bath house that I'm fixing up, and planning to rent.   I'm building an addition to add a third bedroom and bath (master bedroom with onsuite) and a laundry room. 

The house was built in 1950, and much of it is still original.   The kitchen was original.  The bathtub is original.   My question is primarily about replacing the cast iron plumbing.   The bathtub drain was clogged, I snaked it out a couple times and got it draining again.  The house has a crawlspace.

I had to cut open the floor to get into the crawlspace, as the vent openings were not big enough to fit though only 12 inches.  Plumbers must have been midgets in the 1950's?   It does appear the plumbing has been untouched since it was built?

The issue is the drain lines from the tub and show and bath sink connect into a lateral

that tee's into the toilet drain that goes into the sewer downpipe.  It looks like this lateral run has settled over the 50 years and is now LOWER than the tee, and I think this is where the tub drain was clogged.   I'm thinking this is just going to clog up again as the drain isn't 'draining', but has a low spot.   So I'm thinking I need to just replace ALL of this cast iron pipe while I'm rehabing before renting it, and having to fix a bigger problem in the future.

Or should I just leave it be and fix any issue when it happens?   I will need to tie into the stack for the addition's plumbing, so I'll need to get in there already, but don't need to replace all of it.