I have a web app I built using a template but now want to build it using web app builder. Is there a way I can assign my new web app the old url so I wouldn't have to redirect any links I have setup or notify people of this change?
No that is not possible.
When a new item is created in ArcGIS Online, a new unique ID is created. The ID can be seen in the URL on the Item Details page. For example:
The ID is unique across all of ArcGIS Online and cannot be re-used once it's been assigned to an item.
You will need to either modify your existing application to continue to use the same URL, or accept the new URL and replace the old URL with the new one going forward in any other places where you may have linked to the original application.
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