Command to remove computer from non-existant domain

Try netdom remove computername /Domain:domain /UserD:user /PasswordD:* /Force

Type 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:*

Again type netdom join /? for help with the command usage.