-----Description----- 
This task requires writing a Lean 4 method that converts all uppercase characters in a given string to their lowercase equivalents while keeping the other characters unchanged. The output string must have the same length as the input string.

-----Input-----
The input consists of:
s: A string that may contain both uppercase and lowercase characters.

-----Output-----
The output is a string:
Returns a new string where every uppercase letter has been converted to lowercase, and every non-uppercase character remains exactly as in the input.

-----Note-----
There are no preconditions; the method is expected to work for any non-null string.