You may want to use a brushless motor with an existing 3-in-1 controller without converting the helicopter to separate electronics. There are a few different ways to do this.
Note: A 4-in-1 controller is the same as a 3-in-1 controller with a receiver, so these techniques will work with a 4-in-1 controller as well.
A brushless main motor ESC can be added to an brushed 3-in-1 controller by simply using a Y-lead servo connector on the throttle channel. This will allow both the 3-in-1 and brushless motor controller to receive the throttle signal. This will not work for a brushless tail motor because a Y-lead on the rudder channel will bypass the revo mixing capability of the 3-in-1 controller.
A PowerZone adapter board converts a brushed motor signal back into a servo signal (variable width pulse) so the brushless ESC can plug into the PowerZone. This works for a brushless main and tail motors, although a Y-lead is much cheaper for a brushless main motor. However, if the 4-in-1 board has both the receiver and controller on a single board, then using a PowerZone may be easier than trying to trace the receiver's throttle channel to connect a servo connector.
Search the RCgroups micro heli forum for more info on the PowerZone.
If you are using a six or more channel transmitter, then channel 5 should be unused when using the 4-in-1 board. You can slave the unused channel 5 to the throttle channel (usually channel 3) and connect the brushless motor ESC to channel 5 on the receiver.
You can add a heading hold gyro to a brushed 3-in-1 controller by using a heading hold gyro (without remote gain), and a tail ESC. You will need to rewire the electronics as follows:
The throttle channel will go to the 3-in-1 board, and the main motor will still be controlled by the 3-in-1 board.
The rudder channel will now go to the heading hold gyro, and the tail motor ESC will plug into the heading hold gyro.
Basically, you will only use the 3-in-1 board as a main motor ESC, and all other functions will be handled by the other components.
If you are using a brushed 3-in-1 controller, then you can use a PowerZone board to convert the brushed tail motor output into a servo signal, then you can plug a brushless motor controller into the PowerZone board.
Otherwise, if you are already using separates, then just replace the brushed tail motor ESC with a brushless tail motor ESC.
If you are using a heading hold gyro and you replace a brushed tail motor with a brushless tail motor, then the tail will probably wag very badly because the brushless tail motor will generate more thrust than the brushed tail motor.
If this occurs, you will need to trim the tail rotor to reduce the tail rotor thrust. You should trim the tips of the tail rotor instead of the trailing edge of the tail rotor, because this will create a more efficient tail rotor (smaller rotor with more pitch is more efficient at generating static thrust than a larger rotor with less pitch).
Be sure to trim the tail rotor in small steps and test the tail rotor thrust by flying the heli to ensure you do not trim too much from the tail rotor.