Erm. Can hack till bare wall. Then can redo piping... a lot of people do this, but this will cause your wall to be like extra 2inches thick. and not sure if the tiles can adhere to the wall better since there is still old cement left.. best to hack out till you see red bricks or cement bricks IMO.

Oh.. Why he refuse? Did you discuss with him before hand? Or if its additional work? Did he quote the piping works also before he started work or was it after his work commenced? Sorry, you meant the WC 4" outlet? Usually can change if the person knows how to. lol. If he says it cant be changed then means either he is lazy or just inexperienced. If currently its iron pipe, cast iron actually, then you can only change partially, not the entire length to the manhole, that will cost a bomb to do, not advisable unless its leaking at various places. Also see how much he charges, if its very cheap, then thats what you will get. Usually for whole toilet replacement can cost around 8k++, all in, hack, remove, re-pipe, plaster, render, waterproof and tiling. Materials wont cost a lot, labor is expensive these days, IMO la.. lol