Currently there is no way to downgrade from Enterprise to Standard. It is possible to do a one way upgrade of Standard Managed AD to Enterprise Managed AD, though you'd need to open a support case to request this.
Unfortunately, it is also not currently possible to change the primary region after directory creation. The service team has been been made aware of this request and they work on feature releases based on customer request, so if you happen to work with an Account Manager and/or Solutions Architect, be sure to let them know you'd like to add your +1 on the request for visibility.
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