Service Desk 7.3 and up
I have created a new "Before Save" calculation. Is there a way that I can have this calculate on the existing Incidents and not just the new ones?
Yes, you can modify the calculation type to be "Scheduled" and create a schedule to run the calculation. This will update all existing Incidents that are in a none closed status.
You can then change your caluclation back to a "Before Save" calculation.