They can't patch out booting into RCM. To be more accurate, RCM is a feature of the Tegra chip that the Switch uses that comes built-in. It acts like a safety net at best for developers so to be more accurate, there is no reason for them to remove RCM.
What they are fixing in the new hardware revision is the flaw that turns payloads being sent that the Switch turns into properly signed code due to an overflow error.
Well you can but SX OS does not let you run games that have higher firmware requirements. If you want to play games with higher firmware requirements, follow this thread on how to update without burning fuses: https://gbatemp.net/threads/how-to-...nofficially-without-burning-any-fuses.507461/
This way, if you have a NAND dump that matches the current number of fuses burnt, you can restore it which is effectively the same as downgrading