Changing to lower case from n position to end

I'm trying to rename multiple files by changing to lowercase from the n position to the end of file name.
Ex. n=9
from

AaBbCcDdEeFfGg.txt
AaBbCcDdEeFfGgMmNnRrSs.txt
AaBbCcDdEeFfGgMmNnRrSsTtZz.txt

to

AaBbCcDdeeffgg.txt
AaBbCcDdeeffggmmnnrrss.txt
AaBbCcDdeeffggmmnnrrssttzz.txt

How can I do it?

Easiest way is to use a simple script:

n = 9;
name = item.name;
return name.substring(0, n - 1) + name.substring(n -1 ).toLowerCase();

Reply to #2:
Thank you very much. It works fine.