I am sure David Satz could execute a precise, all encompassing narrative of the differences.
In the meantime I try a Klutz version:
The central difference between the U87 and its successor U87A was the introduction of a DC-DC converter in the new model (the 'i'-suffix identifies an XLR connector, which all U87A and most U87 have anyway.)
What is a DC-converter? It is a circuitry ,situated in the space where the battery compartment of the old U87 was, that increases the phantom power's available 48VDC to 60 VDC, and thereby polarizes the capsule voltage with 12 more volts than in the U87.
The extra volts available at the capsule increase the output of the mic by about 10dB. The side effect of the output improvement is a slight reduction in headroom (distortion from very loud sound sources) by 5-6 dB.
All other audio and EQ parameters of the two mics remain the same.
One other advantage of the DC converter is that now the U67 capsules and the (identically constructed) U87A capsules, known as K870, are interchangeable.
The old U87 capsule required an isolation washer between the capsule halves, to generate three patterns out of 48V, which made the U67 and U87 capsules not interchangeable.
Kind regards,