The most common and most likely answer to this crossword clue is the 9 letter word LITERATIM
I've seen this crossword clue in The Guardian
L
I
T
E
R
A
T
I
M
Buddy explains!
The straight part
We know that "without changing any characters" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case without changing any characters = literatim.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does LITERATIM mean?
LITERATIM adverb- (of the copying of text) Letter by letter.