Hello,
our company has two Atlassian instances: for example ABC.atlassian.net and DEF.atlassian.net
We have used ABC as our main instance for several years with a lot of Jira projects and Confluence pages. Now we have rebranded back to DEF (this atlassian instance was not used in the past).
So actually we just want to change the product site URL from ABC to DEF, and keep ABC (as described here and here). But this is not possible since the site URL is already taken by us:

Can I delete the DEF instance so the name becomes available and then rename ABC to DEF?
Or should I export/import all content from ABC to DEF?
What do you think would be the best approach in this case? THANK YOU.