Replace element in a nested list

[ichabod801]

A loop version of Gribouillis's solution, if you are not familiar with list comprehensions:

new_list = []
for sub_list in my_string:
    new_list.append([])
    for item in sub_list:
        if item < 65:
            new_list[-1].append(None)
        else:
            new_list[-1].append(item - 12)
my_string = new_list

They both do the same thing, the list comprehension syntax is just way more compact.