Make checklist item about existing data more tangible

doc/development/code_review.md

@@ -156,7 +156,7 @@ See the [test engineering process](https://about.gitlab.com/handbook/engineering
 1. I have confirmed that this change is [backwards compatible across updates](multi_version_compatibility.md), or I have decided that this does not apply.
 1. I have properly separated EE content from FOSS, or this MR is FOSS only.
     - [Where should EE code go?](ee_features.md#separation-of-ee-code)
-1. If I am introducing a new expectation for existing data, I have confirmed that existing data meets this expectation or I have made this expectation optional rather than required.
+1. I have considered that existing data may be surprisingly varied. For example, a new model validation can break existing records. Consider making validation on existing data optional rather than required if you haven't confirmed that existing data will pass validation.
 
 ##### Performance, reliability, and availability