Just for reference, one 'turns' on a lathe. (Lathing is working with narrow strips of wood)
You can have a number of problems turning down a 200 mm disc to 190 mm depending on the material and the type of disc. If the disc is ventilated (ie two faces separated by vanes) then you are going to have to be careful when cutting across the vanes as the cut is interrupted and going too quick can damage both tool and disc. If the disc is cast iron, it can shatter. (But it sounds as if someone is going to do it for you).
Second, you are going to have to undercut the vanes (make them hidden between the two flanges of the disc) because other wise you will get a series of chisel edges that will quite happily chop up anything that gets caught there (like the next cadet if there's an accident)
If the surface of the disc is grooved then you shouldnt have to regroove the disc. The grooves are only there to remove the layer of gas created by the disc pad getting hot and floating the pads off the surface.
The only problem of legality might be if your class rules state that 'manufacturer's parts' must be fitted but I don't think they do. (TKM for example might be another story)
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