Say, you want to transfer the website example.com from account A to B. There is no direct support for this but you can achieve it by cloning the website 2x.
Steps
Mark the website example.com as open source in its branch settings. This is needed so that account B will be able to clone it.
Account B shall clone the website example.com under a temporary domain, for example example-clone.boomla.net. I recommend you use a boomla.net subdomain as then you will have instant access to it (no DNS setup needed, no need to wait for issuing a HTTPS certificate).
Visit the cloned website, verify it works and that you cloned the correct website.
Delete the original website example.com on account A so that you will be able to create a new website on the domain.
Account B shall now clone the website example-clone.boomla.net under the desired domain example.com. (No need to mark it open source this time as it's owned by the same account.)
Again, visit the cloned website, verify it works and that you cloned the correct website.
Delete the temporary website example-clone.boomla.net.