W nd, 12 bytes
(The following is a way that will work (introduced the l instruction), but hasn't been implemented yet.
26N % Repeat the following 26 times:
'Zk % The uppercase alphabet
:a> % Slice the item after the current counter
naz, 176 bytes
The most involved naz solution I've written so far. This program subtracts 13 from every letter in standard input that's greater than M, and adds 13 to every other ...