Change our sharePoint online sites' urls from "https://OLDname.sharepoint.com/" to "https://NEWname.sharepoint.com/"

Still not possible, afaik: https://learn.microsoft.com/en-us/sharepoint/change-your-sharepoint-domain-name
Although there has been a private program for this for years, best contact your TAM (if you have one).

Hi @john john ,

You can't change the tenant.sharepoint.com part of the URL by registering a new domain and define it as the default domain. In SharePoint online you can change only the address of the site within the URL, for example: "https://contoso.sharepoint.com/sites/A" to "https://contoso.sharepoint.com/sites/B". You can't change the domain or any other part of the path.

For Reference: Change a site address

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