Unlocking an HTC HD2 after installing android I recently bought an HD2, which originally runs Win6.5.  But currently has a very modern version of Android installed.

Is it necessary to re-install the original OS in order to unlock the phone?
90% chance - no, 10% chance - yes. 

The Android operating system is designed specifically for each device, which is why different phones get platform updates at different times. You're at an advantage starting with an HTC phone, as they build several Android phones. 

Your best bet is to find an unlock process for an HTC phone as physically similar to yours as possible (physical in terms of hardware; processor, memory, etc.). If this does not work, chances that you will get anything to work is slim.