Currently in Bitbucket, it's not possible to change an existing custom-named deployment environment's type from one type to another.
Also, since environment names can't be duplicated, the only way to currently change the environment type is to delete the old environment and create a new one with the desired type (test, staging or production).

This feature request is to add the ability to change the environment type for any custom named environment in the deployments section in Bitbucket Pipelines's config.