Hi James,
If you would like, you can migrate the Subversion repositories to GitLab. There's no need to, though, because Subversion will remain a supported version control system. If you still want to migrate, but need help, we can discuss the steps here on clo-devel.
is there a mechanism for the case where it would be best to synchronise them with the github repositories?
Sure! Just file the request with the admins :-) Which Github repositories do you want mirrored exactly? Does this mean that the existing Subversion and CVS repositories on common-lisp.net should be regarded as superseeded? [And can they/should they be removed because of it?]