I've noticed nearly all of the Chinese fm rigs use either a RD06HVF1 or RD15HVF1 output device. These rigs have no protection built in so these devices blow in the blink of an eye in cases of bad SWR/Short circuit etc.
The boards look extremely similar in rigs using either output device, I wonder if these devices are interchangeable, ie could a 6 watt rig take the 15 watt device and vice versa?

Both devices are interchangeable with each other, however you would still get roughly the same output power, eg replace the RD06 with the RD15 the output power may only increase by a very small fraction of a dB. To get more power out, the RD15 requires slightly more drive and also different output matching.