I have both old and new and they make an even sounding set. None of mine have nickel covers, but I think that would make them sound better in general, maybe tame the treble a bit.
The only change I'm aware of, which actually affected all SD pickups, is the switch from butyrate bobbins to polycarbonate. My understanding it that was to make assembly easier (they sit more evenly when screwed to the baseplate) and to survive the wax potting process better. A former employee once pointed out that butyrate, being softer, could also squeeze out of shape slightly under tension, changing the geometry of the coil, but whether an average player would detect whatever difference that makes is doubtful.