Why? Going from 2 to three characters is an extra visual clue that something is being added.
Also, I’ve decided I like the red better than the black, because red fits better with the rest of the theme. (orangy) red is used throughout the theme to indicate changes.
Another idea:
Remove the arrow altogether, and put the “42” in red. The text could then be a bit higher, which might improve readability, and it also signifies better what is actually changing.
I really liked the “123” text on the old icon (KiCad V4 I think) which got changed to a simple red line.