Imagine you are tasked with developing a function that takes a string as input and replaces each letter in the string with the letter that follows it in the alphabet. For example, $\text{'}$a$\text{'}$ becomes $\text{'}$b$\text{'},\text{'}$b$\text{'}$ becomes $\text{'}$c$\text{'},..,\text{'}$ becomes $\text{'}$a$\text{'}.$ Similarly, $\text{'}$A$\text{'}$ becomes $\text{'}$B$\text{'},\text{'}$B$\text{'}$ becomes C$,...,$ and Z$\text{'}$ becomes'A. Any non$-$alphabetic characters in the string should remain unchanged.