Adding a config setting should never be dangerous (if it is your system is deeply broken) and should be distinct from changing an existing config setting.
While I can't name anything specific offhand, I feel pretty strongly that I've seen documentation for various things stating that those things check for an environment variable and, if it isn't present, fall back to other candidate names for the same variable.
This makes setting a new variable synonymous with changing an existing one, unless all variables are currently using the highest-priority possible names.
Another architecture with the same effect is that the software will only check a single environment variable, and if not present it will use a default value. That also makes setting a new variable synonymous with changing an existing one.
I’m talking about altering a config which you control - adding to a config is pretty risk free if it is your program, altering a config of course is not.