We were somewhat able to achieve above. This is what we did, created standby gitlab setup with same version, took backupon primary using gitlab-rake restored it on standby server, upgraded gitlab version on standby server to latest, renamed project on old servers along with project path, Imported repos on standby server (one at a time ) and after that also import the access from corresponding old (renamed) repos. and eventually deleted old repo.
Things which remained pending after above are wiki and issues. So this still requires manual intervention.
If any one know any better way of doing these things kindly help.