question on cast iron piping

[Cindy Joseph]

hello I am new to this and  looking at a property with cast iron pipes under the house built in 1957. There are no leaks. Was wondering how urgent is it to change to pvc? Also what would the total cost of replacing pvc look like? I understand that you have to tear up the floors to do this.  Its a a 2/1 duplex about 850 square feet each side. 

Are pipe liners any good? Ive heard mixed reviews on this.