Rednand is no different from an Emunand like the 3ds was, there all emulation based from the actual main system, trust me it can be updated as long as the nands are unlinked.
Yes it can be updated, but you will brick the other copy of the OS due to antidowngrade protection (the highest run system version is saved to eeprom, which currently isn't emulated in rednand (this sentence doesn't imply or deny it will be soon or ever)), so practically speaking it mustn't be updated even if you can