-----Description----- 
This task requires writing a Lean 4 method that replaces every occurrence of a specified character within a string with a new character. The output should be a new string that maintains the same length as the input string, with all instances of the designated character replaced by the given substitute, and all other characters preserved unchanged.

-----Input-----
The input consists of:
s: A string in which the replacement will occur.
oldChar: The character within the string that needs to be replaced.
newChar: The character that will substitute for every occurrence of oldChar.

-----Output-----
The output is a string that meets the following:
- It has the same length as the input string.
- All occurrences of oldChar in the input string are replaced with newChar.
- All characters other than oldChar remain unchanged.

-----Note-----
There are no preconditions; the method will always work. It is assumed that the input string is valid and non-null.