They are not easy to replace with any degree of good sealing or without a lot of cursing or jack hammering. Often the best thing to do is seal the existing hole up, prefeably with grout followed by welding a cap on it, then drilling a new hole through the bund, inserting the pipe and then fitting and welding a slip on flange onto the pipe on both sides, sealing the annulus with some fire proof gunge and then bolting the flange to the wall with rawl bolts or similar
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
