When you are renovating your home, sometimes it is not necessary to get a brand new irrigation system. But sometimes it is. To determine whether you need a brand new system or not is not that complicated. If it is broken or does not work properly or is 20+ years old then I would recommend replacing it. If it is not that old and works fine then the contractor can either work with it or retrofit it accordingly.
Regardless of what state your system is in, it is always best to leave some allowance for the possibility that something may need to be replaced. You have no clue what the buried piping is like, so having a contingency just in case is always best.