Turning down a servo motor shaft. Anything to look out for?

unless the motor is *very* old, i bet it has a rare earth magnet rotor which survives just fine being pulled out of the stator. the tricky part is not to scratch the rotor coating. i have replaced bearings on a few servos, although smaller ones - 1HP and lower. i also turned a straight shaft to a taper on a 400W mitsubishi servo.

i'd also think about the possibility of fixing the motor somehow to the lathe bed and turning the shaft under it own power. the one i modified was free-machining steel, very easy to turn. haven't tried that, but should work, imho

Does the motor have a rear shaft extension (possibly by taking the rear cover off)? If it does then you can consider mounting the motor between centres on your lathe.
I have a drive-centre which would then make the job pretty easy. If you don't have one then consider mounting the motor on centres but running the motor itself to rotate the shaft.

Or, what would probably work, would be to short the motor windings together and drive the motor housing, there will be full motor rated torque available for machining (I think) though the speed won't match.

Yeah, I'd considdered removing the encoder and having a look, there must be something there for the encoder to key onto and I'd like to bet that there's a centre drilled in whatever's back there. That might be a quick and simple answer. Shaft between centres, hold the body still (just so it's not flogging around) and use a drive dog.
Motors haven't turned up yet, I had a quick look at the original Yaskawa (which I suspect the new chinese motors are a copy of) but I'm reluctant to mess with them.