Without going into the technical detail; the previous value made available in conditional start is actually the current value just before any updates are made to the item. When the item is updated, we lose the ability to get the value prior to the update.
To restate the workaround as indicated in the idea; create an additional column in the list or library and update this with the current value of the column of interest. Then when the actual column is updated, the previous value is still available in the additional column. Of course one need not make this additional column visible in the view or Form so the users are not confused.
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Is there a way in Nintex 2016 to send an email "values changed" notification that will summarize only the values that have changed? Example if my form is collecting 5 pieces of information for a deployment and the deployment date was change from 01/01/19 to 02/01/19 after the form was submitted. We would only want the deployment date change listed in the email notification with the previous value (01/01/19) and the new value (02/01/19).