When you update from 2.5 to 2.6 you are unable to change mapping rules for any existing Istio services.

When you try to update mapping rules from integration page, it will fail with Cannot update the staging configuration.

Based on my investigation I think it is because proxy configuration in 2.6 has these new fields which are now required.

      "error_headers_limits_exceeded":"text/plain; charset=us-ascii",
      "error_status_limits_exceeded":429,
      "error_limits_exceeded":"Usage limit exceeded"

These fields are not present in 2.5 which causes proxy updates to fail because these required fields are missing.

You can workaround this by adding the new staging configuration from API with the new fields, I don't think its sufficient enough for Istio because the staging configuration is not visible.