Agreed. You cannot change future incidents (easily) within MDFC. The same can be said for other solutions like MDE and MDI. If you forward these alerts to Sentinel, you can use an Automation Rules or Playbooks to revise an incident. You could also use a workflow automation (logic app) inside of MDFC to change incident severity levels.
If you want to enrich the incident with more information, then you really need to do that in Sentinel. Enriching incidents using automation to add comments is a common automated response in Sentinel.