Current Scenario:

• When customer tries to create a new Dynamics 365 environment, he/she is provided with available URL names from which he/she could select the name as per their requirement.
• However, once the environment is created and if customer wants to change the URL, they are not provided with available URL names from which the URL name could be selected. 
• This is expected behavior.

Expected Scenario:

• If customer wants to change URL name of existing Dynamics 365 environment, he/she should be provided with available URL names from which the URL name could be selected.