KOReader/Plato themselves are unotuched.
NM survives FW updates, and will (barring massive Nickel changes) keep working.
KFMon is disabled after a FW update (to avoid blowing up in fun and interesting ways in case of massive Nickel changes).
TL;DR:
* If you use plain NM, you're good to go.
* If you use KFMon (or the KFMon generator in NM, which is the default setup for the OCP), you'll want to simply reinstall KFMon.
To that effect, as far as the OCP is concerned:
For option a., the OCP installs NM configs named, respectively, koreader & plato, that allow one to switch to a pure NM setup (the action is just commented out to avoid duplicates).
For option b., the install scripts in #2, in addition to supporting the OCP packages, also support the standard, plain KFMon install package.
When in doubt, option c.: reinstalling your preferred OCP package, also works (and won't affect KOReader/Plato settings)
.