If you use code "age" for a question and set a condition at a follow-up question like "{age.NAOK > 18}" and then later change "age" to "agerange", the condition at the follow-up question will be broken. This also applies to all equation questions making use of the old "age" question code.
When setting conditions using the condition editor, there always is a warning if you try deleting a question which is used for conditions at follow up questions. This is missing when using EM based conditions.

We also have a check if a question code you are trying to use already exists. Maybe something similar could be used when changing question codes. Just load all relevance equations and all equation questions and if the old code is found in there, throw a warning.
Alternatively, maybe we could check if the survey logic file shows X errors prior to such a change and X+N errors afterward. That would also indicate that something bad happened.