I understand that you can buy Oz coded ELT's from the US. I understand that if the mounting cradle to the fuselage needs to be changed, then an EO is required (what a rort). Many of the old 121.5 ELT's ran only 2 wires to the remote switch, but the 406 beacons require 3 wires. Therefore some installations require new wire from the ELT to the cockpit switch. In some aircraft this requires removing a fair bit of the interior which is time consuming (expensive). If you can get a 406 ELT that will use the same mounting tray as the old one and you have 3 wire cabling to the cockpit switch, then I believe its just a matter of changing units & antennae.
In Australia the prime method of picking up 121.5 signals has always been overflying aircraft. Read this:
CAN IT EVER BE APPROPRIATE TO MONITOR 121.5 EN ROUTE?. The airlines and many private pilots monitor 121.5 in cruise with otherwise unused radios. All of these guys will still receive the signals.