Need to modify the automatically created Lambda Function ( s3 trigger with existing )

Serverless Framework
,
1 
functions:
  functionName:
    handler: handler.handlername
    name: Function-Name
    vpc:
      securityGroupIds:
        - ${env:SECURITY_GROUP}
      subnetIds:
        - ${env:SUBNET_1}
        - ${env:SUBNET_2}
        - ${env:SUBNET_3}
    events:
      - s3:
          bucket: ${env:BUCKET_NAME}
          event: s3:ObjectCreated:*
          rules:
            - prefix: Prefix1/
          existing: true
      - s3:
          bucket: ${env:BUCKET_NAME}
          event: s3:ObjectCreated:*
          rules:
            - prefix: Prefix2/
          existing: true 
    destinations:
      onFailure: ${env:SNS_ARN}

we have a application that will trigger a lambda function based on two Prefix of S3 object creation events. We can modify the lambda function Name, IAM role name and VPC configurations that we intent to create but we cannot modify the autogenerated lambda when we used the “existing: true” option which would be useful for existing S3 objects.

Is there an easy way to do this or a possible workaround ?

Thanks