You've all been super helpful and I thank you for that! I have another question though

I'm modifying an existing program change template. Seems simple enough right?
The issue is that I actually want it to control 16 banks each filled with 16 tracks.
For example, the button on the left would control banks 1-16. Then, the button on the right would control tracks 1-16 on the right.
It's basically just a pattern change using program changes (I know that right now this exceeds 127 - I'll deal with that later once I figure out how to access the other half

The first issue is this:
Since I added 16 buttons there is now overlap. For instance I can get to program # 15 but either hitting 0 on the left and 15 on the right OR 1 on the left and 5 on the right. See what I mean?
Any ideas how to change the code so that this does not happen? I can't think of how I would do this..
The next question is this:
How can I change the left buttons to display a letter (A-P) instead of #s 0-15? Is that even possible?
Any help greatly appreciated. Template attached!