there is no way you will damage anything in the U87A (or our other microphones) by operating any of the switches, with phantom power connected. Or, plugging / unplugging the mic with phantom power switched on.
Still, to protect your ears and your headphones / loudspeakers it is very helpful to pull down the faders when monitoring or amplifiying. When plugging / unplugging phantom, or switching patterns, we have something like a 48V or 60V signal impulse. Compare this to typical mic levels of 1microVolt to 2Volts, and you'll realize this can be rather loud. But it doesn't do any damage inside the mic. The U87 has been around 50 years now, so I think we can assume it was properly designed ;-)
Best regards, Martin Schneider / Neumann Mic. Development