[Mondial]

Sounds like a good plan.

I am not sure why there are no voltages listed for Q405, but from a look at the circuit it seems there should be about 3.5 VDC on the base and 24 VDC on the collector. The emitter voltage will vary depending on whether a stereo signal is being received. With a mono signal and stereo beacon off, there should be about 5.6 VDC on the emitter. With a stereo signal and stereo beacon lit, the the emitter voltage should be around 2.8 VDC. The difference in voltage is produced by R414 which supplies voltage from the beacon lamp circuit (through the lamp) to Q405's emitter, shutting it off in mono.

If your beacon is still lit after removing Q404 and Q406, then the only apparent path to ground for the LED is through R414, which may be passing enough current to light the LED. The R414 current would not be sufficient to light the original bulb, so the LED conversion may be at fault as far as the always on stereo beacon.