One easy solution is to make a new arm board plate with threaded holes for the Triplanar mounting that attaches on top of the existing arm board. You can see what these types of things look like in my system pictures
The challenge will then be to attach it to the existing arm board using only the Rega hole without protruding above the surface level. I'd imagine a countersunk area around the hole allowing a nut to lie flush with the upper surface? I also suspect trying to fix the plate using only one large central bolt is not the most stable design
The other downside of this approach is that you are adding height to the mounting which may preclude achieving ideal VTA
if if I was you I'd drill new holes in your existing board 😏
The challenge will then be to attach it to the existing arm board using only the Rega hole without protruding above the surface level. I'd imagine a countersunk area around the hole allowing a nut to lie flush with the upper surface? I also suspect trying to fix the plate using only one large central bolt is not the most stable design
The other downside of this approach is that you are adding height to the mounting which may preclude achieving ideal VTA
if if I was you I'd drill new holes in your existing board 😏