Command to remove computer from non-existant domain
netdom remove computername /Domain:domain /UserD:user /PasswordD:* /Force
netdom remove /? for the full command usage. The /Force option is what you're looking for. Per the help:
Forces the unjoin of the machine from the domain even if the domain is not found or does not contain the matching computer object.
To join the members to the new domain:
netdom join computername /Domain:domain /UserD:user /PasswordD:*
netdom join /? for help with the command usage.