Given that you have galvanised steel piping and are installing something prone to eventual failure in an inaccessible location, use whatever works and don't worry be happy. Sorry if that comes across as snide. At the least, install good components with suitable-for-your-water materials in a way to permit future repair. You could transition the respective metals to a CPVC union and avoid galvanic reaction.

How old is the galvanized? I would remove as much of the galvanized as needed and replace it with something else.

Normally my gut reaction is "if it ain't broke, don't fix it" but in this case we could also say that "not everything that glitters is gold". What we know for certain is that one of the three pipes is showing severe evidence of corrossion that is consistant with the type of corrossion associated with direct contact between concrete and pipe. We can further assume that the concrete floor is 4" to 6" thick. What we don't know is what is the condition of this pipe deeper in the concrete or on the underside of the concrete where this pipe exits. From this line of reasoning we can extend it to the other two pipes as well. It is quite feasible that the visible portion of those pipes is as good as the day it was laid but a mere 1/2" below the surface of the concrete they could also be corroded in the same manner if not worse. From my personal experience I would expect that the section of pipe inside the concrete has by now deteriorated to a point where the galvanized coating is completely gone and what remains is a black section of steel that is brittle and crumbly. In the modern high tech world we often use the non-invasive technique of inserting a camera in a pipe to visually inspect the interior condition however galvanized iron pipe in a DWV system presents a unique obstacle. They did not have DWV grade fittings for galvanized iron pipe so they used the common pressure pipe fittings. This means that all tee's and elbows have sharp bends with no radius curve so a snake or camera will not go through the pipe. Now getting back to Bppark's original question, do we jackhammer the floor and replace all the pipes or do we just replace the section that we know is bad? The first step is to get a fair estimate of how much concrete we would need to break out. We already know that we must break out the section where the pipe is corroded so we begin there. Once the concrete is broken out we can actually see what is under the floor. It is quite possible that all pipe under the slab is cast iron pipe and the transition from galvanized to iron is taking place either in or just below the concrete. In this case we would only need to change the galvanized pipe so the breakout would be localized to the three pipes. If we find that the pipe under the slab is also galvanized we must then try to estimate what the pipe layout is to determine the extent of breakout necessary. Let us begin by determining where the "main drain" runs under the floor. Code requires that all structures must have a "Main Cleanout" at the point where the "house sewer"(line from the municipal sewer or septic tank) meets the "House main drain" (The main drain line inside the structure). For structures with a basement the main cleanout is generally within 3' of the basement wall where the drain exits the structure. Typically a municipal sewer is in an easement parallel to the street so the main cleanout would be somewhere along the front wall of the structure. For septic systems if you know where the septic tank is located the main cleanout would be just inside the basement wall on that side of the house. Code also requires a cleanout on the upstream end of the main drain. It may be at the base of the main stack or it may be a separate cleanout on the floor on the opposite side of the structure from where the main cleanout was found. Code also requires that we must have a cleanout at each change of direction greater than 45degrees so for the moment let us assume that the lines under the slab are run as straight as possible to eliminate the need for additional cleanouts. It would also stand to reason that the installer would want to save labor and material cost by making any branch runs as short as possible. We could then stretch a string or imagine a straight line between the to cleanouts to determine the location of the main drain, then imagine a line from where the pipes in question enter the floor for the most direct route to the main drain. If these distances are relatively short it might be cost effective to begin digging but, I personally hate to indescriminately make a long cut that might hinder the structural integrity of a perfectly good slab so here is where we begin considering alternatives. To this point we have made an educated guess where the branch from the line in question attaches to the main drain. Rather than rely upon a guess we could have a camera run from the upstream cleanout down the main drain line. When we visually confirm the location of the side tap we can determine the exact location by measuring the length of camera line from the cleanout to the tap. With our string stretched between the cleanouts we can then measure that same distance along the string on the top of the slab to determine the exact point of connection. Once we know the exact layout we then have a couple options. 1.We could cut out the concrete and replace the entire run of galvanized pipe with ABS or PVC. 2.All of the fixtures being served by these drains are well above the elevation of the slab therefore we could opt to use ABS or PVC and run the drains above the floor to a point where we can conveniently tie them into the main drain. We could then make a small cutout at the point were the existing galvanized enters the floor, cut the pipe off and fill the cutout, then make another small cutout at the point where the existing pipes tie into the main, cut the pipe and cap the openings in the main, thereby abandoning the old galvanized iron with a minimal intrustion into the slab.

Did you use rigid electrical conduit or galvanized pipe intended for plumbing? The latter is not allowed. If you paid an electrician to make the change, is it still not passing inspection? If it has passed, why bother with the investigation? Surely the AHJ isn't going to give you $1200.