Alert new customers on the sign-up page that they cannot change the subdomain (e.g. xxxxx.onmicrosoft.com) after signed-up tenants
There have been lots of customers who created the tenant subdomain (e.g. xxxxx.onmicrosoft.com) tentatively or just spelled mistakenly when signing-up and tried to change it later.
Or many customers created the subdomain before their company names have changed, then realize they have to create new tenants just because they cannot change the existent subdomain.
Therefore, it would be the best that there is an alert for new customers on the sign-up page to let them know they cannot change the subdomain after signed-up, if it's impossible to change the existent tenant subdomain (e.g. xxxxx.onmicrosoft.com) both now and in the future.
Without that caution on the sign-up page, customers get confused later and they give up using different subdomain or creationg new tenants instead even if they do not want to do.
It's increasing unsatisfied feelings with Office 365 among these customers.