New toilet-wooden floor:
If you have a wooden floor underneath your toilet you can either get under the floor directly or cut a hole in the floorboards and gain access that way. Either way you are in a position to easier adapt the pipe to bring the new PVC pan connector up through the floor center of the new pan spigot. If the existing drain is clay grab a felt tip pen and mark a line around the barrel of the pipe below the bottom point of any offset made by bends you may need to use to give yourself the correct center for your new pan. Then take a cold chisel and with a hammer strike the chisel against the clay pipe. Then simply do so 1/4 of the way around the barrel and then 1/2 way round and 3/4 the way round and then in-between these blows. Going around and around until the pipe cracks completely all the way through. If it breaks unevenly you can tidy it up by nibbling away with a large shifting spanners jaws. A rubber earthenware to PVC coupler will complete this joint being done up with large stainless steel clips with a screwdriver to seal after you have made up your offset of bends as required. To finish off the top end through the floor. Just let the new PVC 4-inch (100 mm) pipe rise up past the floor level and cut it off later flush with a carpenters wood saw and glue up the finishing pan connector collar which will fit inside this pipes interior bore.
In cases where this riser is fairly short and hunched in concrete below the floor above the dirt of the ground. Snap off the pipe at the point where it touches the concrete and use a PVC fitting designed to fit inside the clay pipes internal diameter. And either use a masonry silicone to make it airtight or use a mix of 6 to 1 sharp sand and cement mortar to create a seal around this slip socket and the existing concrete.