How to install offset flange for cast iron pipe set in concrete slab

My husband and I are currently renovating one of our bathrooms and have encountered the apparently standard problem of wanting a 12" rough toilet that we really like, but have a 10" rough in the floor. Aside from wondering endlessly why any plumber would install a 10" rough in modern times, we have moved on and are now wondering if we can install an offset flange to the existing cast iron soil pipe which is set into the concrete slab. Can someone outline the procedure and pitfalls? Can someone explain why our plumber hasn't offered this as one of the solutions but skipped directly to advising us to order a 10" rough toilet (which unfortunatly don't come in very many pleasing shapes)?

From what I read, the offset flanges are more prone to clogging. Is the pipe 3" or 4"? I don't think you'd want to use an offset with a 3", but I'm not sure. Since you'll have to break some concrete to put an offset one in, if you really can't find a 10" that you like (Toto makes some, the ones with the skirt - I have 2 - are the same toilet, you just use their special adapter for the rough-in you have 10, 12, or 14" and no, it doesn't work with other toilets), then you could have them break up some more of the concrete, move the pipe and go from there. My unprofessional opinion.

I really recommend one of the skirted, G-Max Toto's as the best solution. Just don't forget to deal with the water supply by using the correct, wider rough-in or a Dahl's offset valve.

offset

Cast iron offset collars are very difficult to install unless you use one that has the oval opening, and those are almost guaranteed to cause stoppage problems. The "good ones" if there are any are a sort of straightened out "Z" and they require that the vertical pipe be fairly long so it can be cut off about 3" below the floor and still make a joint. Then you have the job of trying to pack oakum into the part of the joint that is under the offset, and then sealing the lead after pouring. 10" roughins are usually the result of bad measurements or bad measuring by someone.