https://goo.gl/photos/A6G2srpiTrJdwxwUA
The pin on the left is a classic medeco pin. The one on the right is a biaxial. The primary reason for the change is because for a key to be restricted, it must be patented. To patent something it must be unique. It can be similar to another design but it must have at least one unique feature. The patents medeco gets are the longest ones available, 20 years. So medeco makes a change every 20 years for the purpose of keeping their key blanks off the public market.
The classic pins often had very faint or no false gates in the key pins. Notice that this one has no false gate. Also, the true gate only goes about 70% of the way down the pin. The retainer tab that keeps the pins from rotating too far is located in a perpendicular position. The main point of focus for the discussion though is this: notice how the tip of the pin has two flat sides culminating in a chisel point at center. All medeco classic pins will have the chisel at center.
The right pin is a biaxial. You can see the true gate to the right of my pick tip. The false gate is being pointed to. The gates go all the way through the pins on biaxial medecos. They also sometimes serrate the key pins. The retainer tab is located linearly and opposite the sidebar gates. Here is where the biaxial comes in: notice how the chisel tip of this pin leaves a large flat surface on one side and almost none on the other. When installed in the lock, the flat surface on the pin pictured would be to the rear of the lock. That would make this pin a 'fore' cut pin. An 'aft' cut pin would have the large flat portion towards the front of the lock. A center cut pin would look very similar to a classic medeco.
Three only thing this effectively does is allow medeco to obtain another 20 year patent on keys, and it changes the way the keys interface with the pins. You could have a classic key and a biaxial key with the same depth cuts and angular cuts and they wouldn't be interchangeable because the classic would have all 'center' cuts and the biaxial has the 'fore' and 'aft' cuts.
From the perspective of picking the lock, the biaxial tends to be easier because we have a larger surface area with which we can manipulate the pins rotationally. Hope that helps.