It always sucks to have to give a client some bad news; but hopefully this can serve as a warning for folks to be careful about buying SM57's from unreliable sources to send them in for upgrading. These SM57's looked absolutely authentic through and through... packaging, metalwork, mechanicals, weight, feel, paint job, print literature, everything... until I extracted the 'transformers' only to find that they are fake empty bobbins not connected to anything, just designed to look authentic under the glue inside the mic body. and the wires ran straight through from the moving coil to the XLR. I believe these mics do work out of the box, but sound more bright and nasal than a real SM57. And they cannot be modified. They are built in such a way that the moving coil element cannot be loaded with a transformer... it has to run transformerless if you want to get anything out of it at all. If you want to be able to identify these without taking them apart, you can do so with a meter. A real SM57 will have an output XLR impedance (measuring pins 2 and 3 with an Ohm Meter) of about 30-50 ohms. These fakes will measure about 600 ohms, give or take, because you are measuring the impedance of the moving coil element directly.
If you see an SM57 being sold as 'new' for anything less than $99, and esp if its being sold on eBay or Reverb or anywhere out of Asia; I would say avoid that sale. Pick up a new one from Vintage King or Sweetwater, its frankly worth the peace of mind.