Some groups are renamed after updated version because group name become reserved.
How can i know which words are going to be reserved in the new release, Do have documents?
Yes, the list of names is in https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/path_regex.rb. Documented a bit in https://docs.gitlab.com/ce/user/group/subgroups/#creating-a-subgroup, but could be in a better place.