Our shop just completed a conversion on a m3 super 90 that did not need the rethread modification. At this point I can not tell you which ones will need the rethread and which ones will not. I believe that the later (older) ones will need the modification. Best advice I can give at this point is pull the original tube and try to fit the 166 tube. If it will thread into the stock you don't need the mod, If it is to large give us a call. You will have to clean the threads in the back of the receiver when you remove the original tube, this is very important. There will be excessive thread locker in the threads that could cause damage to the threads on the stock or the tube (CLEAN THE RECEIVER THREADS). Also a note on removing the original tube; use heat at the back of the receiver where the tube meets the receiver, we do not recommend using any type of fire (i.e. no torch). We use a smd heat gun and set the temp at 350 F.
As we figure out more about the conversion we are sending all our findings to heckler&kochp2000. He will keep everyone informed.
sorry to be so long winded, hope this helps.