2A2 disc swingarm on 81 Yamaha xs400se frame?

[Firestone]

Is it a straight swap with no issues or does the swingarm need some modifications?

I am one of the new guys and still pretty dumb

 

[motoTrooper]

If you're going to put a disc brake swingarm on a bike made for drum brake you may have to remove the old drum pedal mounting pivot and stuff and fabricate a frame mount for a rear master cylinder and push rod attachment.