"If you have already created an account with the username you want to use for your community, you can convert the account.If the account has any entries, they will need to be deleted. While logged in as your personal account, proceed to the conversion page (http://www.livejournal.com/community/settings.bml?mode=create) and enter the username and password for the account you would like to convert. The account that you are logged into when you do this will become the maintainer account. The rest of the page can be used to define various settings for the new community."
It'll push people away from creating a personal journal first, and we can make it support policy to provide this info to people who say they slipped up in the meantime. After a month or two we can finally kill off ?mode=create and just have people create /communities/ as communities, first.
Any objections to doing this?