Thanks for your quick reply. As a matter of fact, I didn't try anything yet. I was reading through the API docs, and read that POST was not allowed for Server Monitors and EC2 Monitors, thereby assuming I could not update them through the API. I think that assumption was wrong, and that it can be done with a PUT.
Updating them in bulk to change the actions is the preferred way, I think, to reduce the amount of API calls. The only thing bothering me right now is determining the monitor_id for each monitor in order to be able to update them. We need to match EC2 instance names with monitor id's at site24x7 side. I don't see any other way right now then to pull the whole monitor list json and parse it.
For type EC2INSTANCE, one could then match the instance id on AWS level with the instance_id on site24x7 level and for type SERVER match it with conf_instance_id. It's doable, just quite some work :)
Side question: for automatically added monitors like EC2INSTANCE and SERVER through client installations, is it possible to add a default action?