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There's no way to undo the upgrade. If you upgrade to Easy References and don't like it, you can "revert" to a previous version of your application that was not using Easy References and continue using the older version of the app builder. However, all applications must eventually upgrade to Easy References.
Do
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Easy References work with imported cases?
Yes, but you may see warnings. You can directly type in the case property as #case/<property_name> to reference a property that is not in the app structure but was imported via the case importer. However, if that property is not used anywhere else in your application, you will see an "Unknown question" warning. This warning will not prevent you from building your application.
You can make the warnings disappear by creating a "fake" form in your application that references the imported properties:
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This will not affect your application, but it will enable CommCare's easy references to recognize the imported properties.
Can I reference specific iterations in a repeat with Easy References?
Currently this is not possible. To do this you can use the old style of reference: /data/path/to/repeat/question[1]