This is a great problem, but I think the problem statement could be a bit clearer. It describes the modification rule as "if you modify exactly one character into another character in this word."
If that were true, and the word "ab" were added, the valid modified words would be ["aa", "bb"] because the one letter modification must be a letter "in this word."
Running some tests, it seems like the modification rule is that any character can be any other character in [a-z]. Removing the phrase "in this word" would clarify the problem statement.