I'm looking to generate a list of all of the user stories that have had a change in plan estimate across my workspace for a given date range 1-7 days for example.
How is that structured in a query?
I don't think you can do that with the Apps included with Agile Central. I believe you would have to write your own Custom App, using the APP SDK. You would have to use the "Lookback API" to get the snapshots of the stories and look at each snapshot for the Plan Estimate to see if it has changed. You could start by looking on GitHub repositories for similar apps written by other people. There are quite a few public GitHub repositories that have custom apps that other people have written.