Add docs for changing the explore of an existing chart (#566)

Author: rephus

get-started/exploring-data/using-explores.mdx

@@ -133,4 +133,20 @@ You'll get a nice **Saved chart view** that you can use to share with others.
 
 <Frame>
   ![](/images/get-started/exploring-data/using-explores/saved-chart.png)
-</Frame>
+</Frame>
+
+### Changing the explore of an existing chart
+
+Once a chart is saved, you can switch the explore (table) it is built on without having to recreate the chart. Open the saved chart, open the **Change explore** dialog, pick the new explore, and all field references will be remapped to it.
+
+<Frame>
+  ![](/images/get-started/exploring-data/using-explores/change-chart-explore.png)
+</Frame>
+
+Tick **Also update all other charts using this explore** to remap every chart built on the current explore in one go — useful when an entire model is being replaced.
+
+Fields that exist on the new explore with the same field IDs are kept; anything that doesn't exist on the new explore is dropped, and you'll need to re-pick the missing fields before saving.
+
+<Tip>
+  A handy use case is **migrating charts from one model to another** — for example, when you rename a dbt model or replace it with a new version. Instead of rebuilding the charts from scratch, point them at the new explore and keep the configuration that still applies.
+</Tip>