Qlikview will usually update the fieldnames in object properties and expressions automatically when fields are renamed in the script. This works well when the only script changes are new names. Structural changes like adding or re-ordering fiedsl can make this less effective. So best to do your renames and reload before moving on to other changes.
The Settings, Expressions Overview dialog allows you to search/replace all expressions in your app. This is another way to make changes, or fix up those that were missed by the automatic process.