A difference needs a history
Two versions of a document may differ because of copying, revision, annotation or a missing page. The difference is worth recording, but it does not by itself establish who changed the text or why. First identify which version each source claims to be. An unsigned draft, a signed letter and a later transcript are different objects, even when most of their wording matches.
Worked example: an extra sentence
Suppose a carbon copy contains a final sentence absent from the signed original. One explanation is that the original was altered. Another is that the sentence was added to the copy after the sheets separated. A dated draft or a reference in later correspondence could help distinguish those accounts. Without such support, label the discrepancy and retain both possibilities rather than describing a forgery as though it has already been proved.
Choose the next comparison
Ask which consequence each explanation predicts. If the sentence was present when the letter was sent, a reply might address it. If it was a later private addition, the recipient may never mention it. Neither prediction guarantees an answer, but it directs attention toward relevant evidence. For a fictional cold case, the goal is a supported version history, not the most dramatic interpretation of every visible difference.