first question, why didn't you delete the OLA in the console? Before you can do this, you have to delete the rules on the OLA or you get an error.
If the OLA has ever been used then I'm not sure you can delete it.
However to attempt deleting this you can try the following, first you need to remove any Agreement Rules which reference the OLA.
Then you would need to remove any Escalation Actions you have defined on the OLA.
Lastly you can try to delete the OLA, none of the objects within lifecycle have a deleted flag which means they are a "Hard Delete" due to the database constraints if the record has been used then it will error when trying to delete.
If data integrity is not a concern, you would need to write a script to check for the response level having been used and update the record to a different response level. This will "NOT" perform any recalculation of escalation points which is why I mention data integrity.
Performing the above update is very unlikely to be supported by LANDesk and anything where you update values directly in the database should be tested on a test copy of the database and when applying to live you should have a working backup to restore from if it goes wrong.