#1MinuteTip You cannot change the My Domain URL in Sandbox, Trial or Developer Orgs in Salesforce. This feature was available earlier but this no longer seems to the case.
When you are signing up for a new Developer Org, Salesforce will automatically assign a My Domain URL based on the company name that you provide. For example, when I provided my company name as ‘asagarwal.com Pte. Ltd.’, the My Domain URL that I got was ‘https://asagarwalcompteltd-dev-ed.my.salesforce.com’. If this my domain URL already exists, then Salesforce will simply add a running serial number. For example, https://asagarwalcompteltd2-dev-ed.my.salesforce.com
I reached out to Salesforce Support on Twitter for the same and here are some of the useful URLs for your review.