Depends on how it's structured in the monolith. If it's well encapsulated — eg. one API call to trigger a notification that goes to a job-runner which sends the actual notification, which is the standard way of doing these in the monoliths I've seen — it's going to be similarly simple/hard.

But that was exactly my point: how hard it is depends on the code structure that has no relationship to whether you are in a SOA or a monolith architecture.

Sure, some (bad) patterns are hard in SOA which are trivial in a monolith, but they are bad patterns regardless. What matters is that you replace bad patterns with good patterns.