As far as I have heard they are not requiring the beacons in private aircraft to be changed yet. Many towers and most of the VOR’s still have 121.5 receivers in them. Of course event the 406 beacons transmit on 121.5 so rescue teams can home in on them. With ELT’s it’s the last 100 yards that is the hardest. A lot of automatic reporting systems are on 121.6 we use to use 121.6 for training but had to move to 121.775 for that. Both the 121.5 and the 243 are AM signals. Just like most high tech equipment, the more you need it the better chance it will not work.