If you are going to redesign your bathroom it would be best to cut back the cast iron pipe as far as possible and replace it with PVC piping of the same size. Cast iron piping was used many years ago and at that time we were not using as many chemicals as we do now, for insatance some of the chemicals used to unclog drains has acid in themand this will slowly eat away at galvanized and iron pipes. PVC piping is impervious to most chemicals and is much cheaper and easier repair or place.
I hope this helps you, good luck.