Microsoft has only just implemented (in preview) the ability to change the name/URL of your SharePoint site[0], a feature that has been widely requested for well over a decade.
Atlassian users might have a while to wait yet :-)
Theres a note on the ticket that domain renames can be requested via support and this process does work - I've tried it on my own site :)
I've had some visibility of the internals of this work and it has involved touching a very large number of systems across a lot of teams. If you're interested in this issue, do also follow CLOUD-6999 which tracks custom domains and has a number of updates which are related.
https://jira.atlassian.com/browse/CLOUD-7184
With 740 companies asking for it ;)
Submitted in 2014.