The 12 speed units have slightly more clearance for the inner shift cables. The right side hole is also shaped so that it is difficult to see to make sure that you have threaded the hole properly,

I recommend that you remove the bar lock and then slip the lock up the handlebar. Then you can look directly into the bottom of the lever and make certain that you are threading the inner shift cable into the proper spot. If you do this blind it is possible to thread the cable around the shift mechanism and it will feel like it is in the proper spot until it is loaded and then pulls the end under the mechanism and jams the cable. This is why many of the levers come with pre-threaded shift cables.

Since the new levers use a Torx 25 you can buy that size driver, lock it properly in place and easily thread the lever onto the handlebar lock.

I have now made enough mistakes that I can disassemble and reassemble the shifting mechanism. Hint - it requires NO spring loading and assembles completely by hand.

Do not fall into the trap thinking that it requires any special tools. But it is possible to screw up the plastic shifting mechanism and need to replace it if you're not careful. But you would need to be a real klutz to do that.