In Salesforce, a namespace prefix is a 1–15 character alphanumeric identifier used to distinguish your managed package and its contents from other packages on AppExchange. Once you configure your Developer Edition organization's developer settings and register a namespace prefix, it can't be changed or deleted. This article explains the limitations and the available workarounds.
Once a namespace prefix is created, it can't be changed or deleted. The workaround is to move all your code to a new Developer Organization, where you can setup the desired Namespace Prefix.
You can visit our IdeaExchange web page to promote or create an Idea. This will allow our dedicated team of developers, to implement the most promoted Ideas in future releases.
Please refer to the IdeaExchange post for a suggested change to the behavior of this feature: Cannot remove/edit Namespace prefix, once defined for Managed Apps
See also:
Please refer to the Product Manager's post on the Community Board: How to change the namespace after package creation?
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