No, there was never a change in the minimum number of miles per segment -- it is still 500.
MQM's are based on a minimum 750, but that does not mean you will get at least 750 MQM's. If you fly LUT fares, you will get 375 MQM's, if you fly discounted fares you will get the 750 and if you fly full coach fares you will get 1125 MQM's.