Restore Chamfer or Bevel?

[e30m3]

I need to file down the lugs on one of my watch cases. In doing so, I'll lose the (Chinese) factory chamfer. It's a "Rolex," so the top of the lugs are brushed, and the chamfer/sides are mirror. The lines are very sharp; the chamfer appears to be 45 degrees or so and it's an abrupt transition. How do I restore the chamfer after filing the lugs down? TIA.

[jackflash]

Diamond file would be my preference, taking it slow and referring to a gen:

http://www.metersfirst.com/watches/1967-rolex-submariner-5513-meters-first-unpolished