The "Host:" field of the client workspace specification is optional: it can be changed to the new hostname or even removed. Please be aware that by default, a client workspace specification is tied to a client host. While it is okay to not use the Host: field, sharing a client workspace across client hosts, or between users, is not recommended.
To find the proper hostname, run
and look for the line, "Client host:".
The Host field can be changed by running the p4 client command from the command line. You can also use P4V; the Workspace field in the Open Connection dialogue is optional, and you can go to View->Workspaces, edit the workspace in question, and enter the client host seen from p4 info.