I am told that they went with a different steel in the frames and the bluing didn't wear very well. Most post-64 94s do seem to lose their finish rapidly. There were also some manufacturing cost cutting in the internal parts (the lever ststem) and a gunsmith tells me they are not as sturdy. Not really huge changes and I would use a newer M94 without hesitation but enough to make a lot of oldtimers prefer the pre-1964 guns. I think the changes were made at around serial 2,700,000 if I recal right.
Of course, it was the Model 70 that got the biggest change in 1964. The pre-64 M70s were true Mausers with dual locking lugs and controlled feeding whereas the post 64 guns were push feed like the Remington 700.