Drst42,
The FlexResponse API is something that ALLOWS you to change some things that is part of the Incident itself. In order to change the Severity of the incident that is OUTSIDE of the platform will require you to write a custom API, in java or another program.
I know that you can update the attributes and other fields in the incident, but changing the status might be toiugher and not reccomended.
What I did before for another customer is to update a custom attribute and NOT the severity. Let the severity be set by the platform and then based on the Flex Response API update another custom attribute filed with what you want. That way you can still run reports and search for the new 'tag'.
Your best bet is to look at the Examples that they provide with the FlexResponse API and docs to learn how to do this.
I have done this before, but it requires some really good programming skills.
Hope this makes sense.
If this solves your questions please marked as solved.
Ronak