You can only change the name of the user groups, this by editing MediaWiki:user-identity-box-group-. Add the name of an existing user group after the "-", example: sysop, blocked, founder, etc. (Note: Wikitext does not work, you can only change the name)
For UCP, the system messages have been changed from "user-identity-box-group-*" to "user-activity-group-*" and are not currently editable. There is UserTags and ProfileTags but both have yet to be verified compatible with UCP.
So, in short, you cannot change them at this time.