There is no need to change the audio device in AGEphone as at this moment you indeed need to use BTAudioToggle.exe. For further information about this utility please refer to
this thread. The package you can download there or at the end of this post also contains BTAudioOn.exe / BTAudioOff.exe, but using BTAudioToggle.exe does the job just fine in most cases.
Please go to "Settings - Other" and give AGEphone the path to the .exe file there for "Execute on Connect" and "Execute on Disc." without any further parameters. If your BT headset is paired properly with your phone AGEphone will automatically switch output and input to the device whenever you accept a call. It's probably of note that BTAudioToggle.exe only works with the Microsoft Bluetooth Stack. I hope this helps to solve your problem for now.