Hidden pan connector concrete floor:
When placing a close-coupled toilet pan and cistern with hidden connections underneath onto an existing concrete floor drain position. If the old collar rising up through the floor is PVC then use a carpenters’ wood saw and cut it off flush with the concrete. If the collar rising up through the floor is clay place a tight rag down inside the pipe up stand and working with inside to outside tapping with a hammer break this collar off even with the concrete floor. If the collar is cast steel you will have to grind it out of the way. Then go to your plumbers’ merchant store and buy a rubber ribbed at the bottom plastic pan connector that has a drop over into it rubber seal on the top. Take home any other type of pan connector you can get to also try out (do not try and use a full rubber connector). At the end of the exercise you must clean off the concrete surface and run a masonry grade silicone around the new connection onto the concrete to make sure it is an airtight -watertight seal.
Note: There are also offset pan connectors that you may have to use if the new pans center of its outlet is different to the existing drain collar center from the wall to the center. Best practice is to buy the best fitting pan and cistern you can get that closely matches your existing wall to center of spigot measurement.
Likewise if the existing collar is coming out of the wall. Your new P-trap pan spigot center-to-center measurement cannot be lower than the existing pipe center allowing for the change in slope change in measurement off the floor.