To change membership or posting access of a community at any time, you need to go back to http://www.livejournal.com/community/create.bml and enter the relevant maintainer and community usernames and passwords, chose the new level of access and then click "create community".
To change the maintainer of a community, you would also go back to http://www.livejournal.com/community/create.bml and enter the usernames and passwords of the community and the new maintainer.