We introduce two new concepts which are the $k$ to $k$ substitution and $k$ to $k$ exchange of a letter on infinite words. After studying the return words and the special factors of words obtaining by these applications on Sturmian words and modulo-recurrent words. Next, we establish the complexity functions of these words. Finally, we determine the palindromic complexity of these words.

Sturmian words modulo-recursive $k$ to $k$ substitution $k$ to $k$ exchange complexity function palindrome