Hilchot Shabbat 11:13-14 (Writing in an unusual way, and editing an existing letter)

The Rambam taught that someone who writes two letters on Shabbat is liable, but is exempt for writing only one letter. Today he explains that editing a single letter so that it becomes two letters makes a person liable, as well. But what does the Rambam say about editing a single letter so that it becomes a different letter rather than two letters? The answer is not obvious.