I've something like kailash.sharepoint.com for which the admin site URL will be kailash-admin.sharepoint.com . I want to change that particular admin site URL to something like kailashadmin.sharepoint.com .I thought this was impossible, I've seen one of my customer is having a URL like this, so it is possible in Office 365 SharePoint. I don't want redirecting, I completely want to change it to a new URL without '-admin' in it.
No, SharePoint Online not supported to change Admin Site url.
This is by design with this format:
https://Tenant-admin.sharepoint.com