In this specific instance, you'll only need to reinstall KFMon, as that's the only bit of your setup that needs to get its hooks into a firmware-provided script, as @frostschutz explained
.
That said, reinstalling KOReader wouldn't hurt, because config files & user data are handled sanely
.
(Also in this specific instance, I see KFMon being disabled by FW update as a safety net in the unlikely event Nickel would have a massive behavior change that could trigger all kinds of funky stuff if KFMon were kept active
).